Control system and method for direct fuel injection engine

ABSTRACT

In a catalyst activation control system for a direct fuel injection engine for igniting primary fuel injected into each cylinder, an additional fuel is injected at least once into each cylinder from an early period to a middle period of an expansion stroke of the primary fuel combustion in accordance with the engine operating conditions, to fire the additional injection fuel by flame propagation of the preceding fuel combustion (without re-ignition) for eliminating misfire, so that an exhaust gas temperature can be raised stably to activate catalyst for purification of exhaust gas. Further, any one of the additional fuel injection method and an ignition timing retarding method can be appropriately selected in accordance with the engine operating conditions for economization of fuel. Further, whenever a quantity of heat generated by the primary fuel combustion is not large enough to fire the additional injection fuel, any one of a method of changing a stratification combustion to a uniform combustion, a method of increasing the first fuel injection amount, and a method of interrupting the additional fuel injection can be selected appropriately for protectiing the catalyst.

BACKGROUND OF THE INVENTION

The present invention relates to a catalyst activation control systemand method for a direct fuel injection engine, and more specifically toa system and a method of activating catalyst disposed in an engineexhaust system by increasing exhaust gas temperature even if theair-fuel ratio of mixture is low. Here, in the direct fuel injectionengine, fuel is directly injected (without being mixed with air beforeinduced into each cylinder) into each cylinder and then ignited by sparkignition, respectively.

In the engine in which fuel is directly injected into each cylinder andthen ignited by spark ignition, stratified combustion method isgenerally adopted. In this combustion method, the fuel is injected intoeach cylinder in the later half period of the compression stroke so thatmixture of fuel and air can be stratified and further only a relativelyrich mixture in the vicinity of an ignition plug can be ignited, withthe result that the engine can be driven in a very lean air-fuel ratioto realize a low fuel consumption.

In the stratification combustion method, however, the mixture formationis severely affected by the fuel atomizing characteristics such asatomization rate and atomization angle of the fuel injector. In moredetails, FIG. 1 shows the combustion process in the single fuelinjection. As shown in FIG. 1, when the atomized fuel is injected at awide angle, since the outermost gas is excessively lean, even when sparkignited, the outermost fuel cannot be ignited perfectly, so that thereinevitably exists a non-combustion region. As a result, theconcentration of hydrocarbon (HC) increases in the exhaust gas. Further,FIG. 2 shows the relationship between the heat generation pattern andNO_(x) generation rate, in the single fuel injection. As shown in FIG.2, when the amount of the fuel injection is excessively large, the heatgeneration rate tends to increase in the first half of the combustion,with the result that the gas ignited in the first half is compressed inthe second half and thereby nitrogen oxide NO_(x) tends to be generatedin the exhaust gas. This is because the initial combustion becomesactive.

Further, in the above-mentioned stratified combustion method of themixture, the exhaust gas temperature is largely reduced, as comparedwith the ordinary uniform combustion method. This is because the thermalefficiency is high and thereby the thermal loss is low and in additionthe quantity of air heated per unit fuel is large in the stratifiedcombustion. Consequently, the exhaust gas temperature becomes lower thana lower limit of the catalyst activation temperature (which is usuallydetermined on the basis of the exhaust gas temperature of theconventional uniform combustion engine), in particular in a low-loadengine driving range such as idling. As a result, there exists such apossibility that the performance of exhaust gas purificationdeteriorates.

To cope with this problem (i.e., to increase the exhaust gastemperature), Japanese Patent Application laid-open (Kokai) No.4-183922(1992) discloses such a catalyst activation method that fuel isinjected again in the expansion or exhaust stroke of the engine (i.e.,the twice fuel injections), in addition to the ordinary single fuelinjection, in order to raise the exhaust gas temperature by re-ignitingthe secondly injected fuel, for activation of the catalyst.

In the above-mentioned prior art method, however, since the fuel isinjected and ignited twice for each engine cycle, there exist problemsin that the ignition energy consumption not only increases, but alsothat the possibility of misfire is high in the second ignition.

The reason is as follows:

Since the second fuel is re-injected in the expansion or exhaust strokeand then re-ignited in the exhaust stroke after the primary combustionby the first fuel injection and ignition has been completed, thereinevitably exists a time interval between the first primary fuelcombustion and the second subsidiary fuel combustion.

Accordingly, it is difficult to form an ignitable mixture in thevicinity of the ignition plug due to a drop in the exhaust gastemperature. That is, it is difficult to well control the second fuelcombustion.

In the above-mentioned case, if the second injection fuel is not ignited(the abnormal combustion) the durability of catalyst not onlydeteriorates markedly, but also the exhaust emission performance isdegraded markedly. This is because fuel itself is emitted as it is.

In addition, since the exhaust gas temperature cannot be increasedsufficiently high by only the twice fuel injections when the engine isstarted at a low temperature, it is impossible to activate the catalystat an early stage. On the other hand, it is not advantageous to alwaysinject fuel twice, from the standpoint of fuel consumption, when thetarget exhaust gas temperature is determined relatively low according tothe engine operating conditions.

SUMMARY OF THE INVENTION

With these problems in mind, therefore, it is an object of the presentinvention to provide a catalyst activation control system for a directfuel injection engine, by which twice or more fuel combustion can berealized securely to raise the exhaust gas temperature for catalystactivation, without depending upon re-ignition.

Further, another object of the present invention is to provide acombustion control method for a direct fuel injection engine, by whichthe non-combustion region of the outermost atomized mixture can beeliminated by improving the combustion controllability in a mixture oflean air-fuel ratio, in order to improve the combustion efficiency andthe exhaust gas emission.

Further, the other object of the present invention is to provide acatalyst activation control system for a direct fuel injection engine,by which the catalyst can be activated at an early stage, while keepingthe fuel consumption rate as low a level as possible.

A further object of the present invention is to provide a combustioncontrol method for a direct fuel injection engine, by which the catalystcan be protected from deterioration, even in case of the abnormalcombustion of the primary or preceding injection fuel, while preventingthe exhaust gas emission from being degraded markedly.

To achieve the above-mentioned objects, the present invention provides acatalyst activation control system for a direct fuel injection engine,comprising: engine operating condition detecting means (3, 12, 13, 15,16,) for detecting various engine operating conditions; first fuelcombustion means (14, 51, 52, 53, 54, 55, 56, 57, 58, 7, 11; 54A, 55A,61A) for spark-igniting first fuel directly injected into each of enginecylinders according to detected engine operating conditions; andcatalyst activating means (61, 62, 63; 54A, 55A, 61A) for injecting anadditional fuel at least once into each of the engine cylinders from anearly period to a middle period of an expansion stroke of the precedingfuel combustion according to the engine operating conditions, to firethe additional injection fuel by flame propagation of the preceding fuelcombustion so that an exhaust gas temperature can be raised for catalystactivation.

Here, the engine operating condition detecting means comprises: an airflow meter (3) for detecting a flow rate of air introduced into thecylinder; a throttle opening rate sensor (12) for detecting an openingrate of a throttle; a crank angle sensor (13) for detecting crankangular positions; a coolant temperature sensor (15) for detectingcoolant temperature; and an exhaust gas temperature sensor (16) fordetecting exhaust gas temperature.

Further, the first fuel combustion means comprises: a cylinderdiscriminate sensor (14) for discriminating each cylinder; a injector(7) for injecting fuel into each cylinder; a fuel ignitor (11) forigniting the fuel injected into each cylinder; engine speed calculatingmeans (51) for calculating engine revolution speed (N) on the basis ofsignals outputted by said crank angle sensor (13); crank positiondetecting means (52) for detecting crank angular positions for eachcylinder; operating condition detecting means (53) for calculating abasic fuel injection amount (T_(p) =K×Q/N), as engine load, on the basisof the engine operating conditions (Q, N) detected by said engineoperating condition detecting means and a correction constant (K)determined for said fuel injector (7), to decide any one of an ordinaryuniform combustion for injecting fuel in an early period of an engineintake stroke and a stratified combustion for injecting fuel in a laterhalf period of the compression stroke but immediately before fuel sparkignition; first fuel injection amount setting means (54, 54A) forsetting a first fuel injection amount (T_(i1) =T_(p) ×COEF×T_(s)) bycorrecting a basic fuel injection amount (T_(p)) calculated by saidoperating condition detecting means (53) on the basis of a coefficient(COEF) determined according to the current engine operating conditionsdetected by said engine operating condition detecting means and avoltage correction rate (T_(s)) determined on the basis of a batteryvoltage (V_(B)); first fuel injection timing setting means (55, 55A) forsetting a first fuel injection timing (T_(INJ1)) according to thecombustion mode decided by the operating condition detecting means (53)and the detected engine operating conditions (N, T_(p)); ignition timingsetting means (56) for setting an ignition timing (θ_(IG)) by correctinga basic ignition timing (θ_(BASE)) decided by the detected engineoperating conditions (N, T_(p), coolant temp); fuel injection timingcalculating means (57) for calculating the set fuel injection timing interms of a crank angular position relative to a reference crank angle,and outputting the calculated fuel injection timing signal to theinjector (7); and ignition timing calculating means (58) for calculatingthe set fuel ignition timing in terms of a crank angular positionrelative to the reference crank angle position, and outputting thecalculated fuel ignition timing signal to the ignitor (11).

In the first embodiment of the catalyst activation control systemaccording to the present invention, the catalyst activating meanscomprises: second fuel injection discriminating means (61) fordiscriminating whether the current operating conditions lie within arange where a second fuel injection is necessary or not on the basis ofthe detected engine operating conditions and for outputting a commandindicative of a second fuel injection; second fuel injection amountsetting means (62) responsive to the second fuel injection command forsetting a second fuel injection amount (T_(i2)) fired by the flamepropagation of the first fuel combustion, the set second fuel injectionamount (T_(i2)) being applied to the fuel injection timing calculatingmeans (57); and second fuel injection timing setting means (63)responsive to the second fuel injection command for setting a secondfuel injection timing (T_(INJ2)), the set second fuel injection timing(T_(INJ2)) being applied to said fuel injection timing calculating means(57).

Further, in the second embodiment of the catalyst activation controlsystem according to the present invention, the catalyst activating meanscomprises: fuel injection discriminating means (61A) for discriminatingwhether the current operating conditions lie within a range where aplurality of fuel injections are necessary or not on the basis of enginespeed (N) and the engine load represented by the calculated basic fuelinjection amount (T_(p)) and for outputting a command indicative ofplural fuel injections; plural fuel injection amount setting means (54A)responsive to the plural fuel injection command, for setting a pluralityof fuel injection amounts (T_(i2), T_(i3), . . . ) fired by the flamepropagation of the preceding fuel combustion, a plurality of the setfuel injection amounts (T_(i2), T_(i3), . . . ) being applied to saidfuel injection timing calculating means (57), the plural fuel injectionamount setting means (54A) being incorporated in the first fuelinjection amount setting means (54); and plural fuel injection timingsetting means (55A) responsive to the plural fuel injection command, forsetting a plurality of fuel injection timings (T_(INJ2), T_(INJ3), . . .), a plurality of the set fuel injection timings (T_(INJ2), T_(INJ3), .. . ) being applied to the fuel injection timing calculating means (57),the plural fuel injection timing setting means (55A) being incorporatedin the first fuel injection timing setting means (55).

Further, in the third embodiment of the catalyst activation controlsystem according to the present invention, the system further comprises,in addition to the first embodiment, target exhaust gas temperaturesetting means (71), when the second fuel injection discriminating means(61) discriminates that the engine operating conditions lie in a rangewhere the catalyst cannot be activated, for setting a target exhaust gastemperature; temperature rasing method selecting means (72) forselecting any one of a twice fuel injection method and an ignitiontiming retard method according to the set target exhaust gas temperatureunder consideration of fuel consumption rate; and ignition timingretarding means (73) for retarding the ignition timing decided by theignition timing setting means (56).

Further, in the fourth embodiment of the catalyst activation controlsystem according to the present invention, the system further comprises,in addition to the third embodiment, an exhaust gas temperature sensor(16); comparing means (74) for comparing the target exhaust gastemperature set by the target exhaust gas setting means (71) with anactual exhaust gas temperature detected by the exhaust gas sensor (16);and the second fuel injection amount setting means (62) and the ignitiontiming retarding means (73) adjusting the second fuel injection amount(T_(i2)) and the ignition timing retard rate (θ_(RD)), respectively onthe basis of the difference between the target exhaust gas temperatureand an actual exhaust gas temperature under feedback control.

Further, in the fifth embodiment of the catalyst activation controlsystem according to the present invention the system further comprises,in addition to the third embodiment, an exhaust gas temperature sensor(16); catalyst activation discriminating means (75) for discriminatingwhether the exhaust gas temperature reaches a temperature at which thecatalyst can be activated, on the basis of an actual temperaturedetected by the exhaust gas temperature sensor; third fuel injectionamount setting means (76) for setting a third fuel injection amount; andthird fuel injection timing setting mans (77) for setting the third fuelinjection timing according to the target exhaust gas temperature.

Further, in the sixth embodiment of the catalyst activation controlsystem according to the present invention, the system further comprises,in addition to the first embodiment, cylinder pressure detecting means(64) for detecting a cylinder pressure generated by the first fuelcombustion; heat generation rate calculating means (65) for calculatingheat generated by the first fuel combustion on the basis of the detectedcylinder pressure and the crank angular position detected by the crankangle sensor (13); abnormality discriminating section (66) fordiscriminating whether the first fuel combustion is normal or not on thebasis of the calculated heat generation rate; and combustion modechanging means (67), when the abnormality discriminating section (66)outputs an abnormality signal, for changing the stratified combustion tothe uniform combustion.

Further, in the seventh embodiment of the catalyst activation controlsystem according to the present invention, the system further comprises,in addition to the first embodiment, cylinder pressure detecting means(64) for detecting a cylinder pressure generated by the first fuelcombustion; heat generation rate calculating means (65) for calculatingheat generated by the first fuel combustion on the basis of the detectedcylinder pressure and the crank angle detected by the crank angle sensor(13); abnormality discriminating section (66) for discriminating whetherthe first fuel combustion is normal or not on the basis of thecalculated heat generation rate; first fuel correcting parameterselecting means (81), when the abnormality discriminating section (66)outputs an abnormality signal, for setting at least one parameter forcorrecting the first fuel combustion; and first fuel correctingparameter setting section (82) for setting the selected parameter to thefirst fuel injection amount setting means (54), the first fuel injectiontiming setting means (55) and the ignition timing setting means (56),respectively, to increase the heat generation rate by the first fuelcombustion in the succeeding cycle, respectively without changing thecombustion mode from the stratification combustion to the uniformcombustion.

Further, the present invention provides a catalyst activation controlsystem for a direct fuel injection engine for spark-igniting first fueldirectly injected into each of engine cylinders, while purifying exhaustgas by a catalyst disposed in an engine exhaust system, wherein anadditional fuel is injected at least once into each of the enginecylinders from an early period to a middle period of an expansion strokeof the first fuel combustion according to the engine operatingconditions, to fire the additional injection fuel by flame propagationof the preceding fuel combustion so that an exhaust gas temperature canbe raised for catalyst activation.

Further, it is preferable that any one of the additional fuel injectionmethod and an ignition timing retarding method is selectively selectedaccording to the engine operating conditions. Further, it is preferablethat when a quantity of heat generated by the first fuel combustion isnot large enough to fire the additional injection fuel, any one of amethod of changing a stratified combustion to a uniform combustion, amethod of increasing the first fuel injection amount, and a method ofinterrupting the additional fuel injection is selectively selected.

Further, the first embodiment of the present invention provides a methodof activating catalyst for a direct fuel injection engine, whichcomprises a first fuel injection setting process and a second fuelinjection setting process including the steps of: detecting engineoperating parameters (S102); calculating a basic fuel injection amount(T_(p)) on the basis of the detected engine operating parameters (S101);setting a first fuel injection amount (T_(i1)) by correcting the basicfuel injection amount (T_(p)) on the basis of various engine operatingconditions (S103, S104, S105); setting a first fuel injection timing(T_(INJ1)) (S106); checking whether a second fuel injection is needed ornot (S152); setting a second fuel injection amount (T_(i2)) (S153) whenjudged the second fuel injection is needed; and setting a second fuelinjection timing (T_(INJ2)) between 30 and 60 degrees in crank angularposition after top dead center (S154).

Further, the second embodiment of the present invention provides amethod of activating catalyst for the direct fuel injection engine,which comprises a first fuel injection setting process and an n-timefuel injection setting process including the steps of: detecting engineoperating parameters (S51); calculating a basic fuel injection amount(T_(p)) on the basis of the detected engine operating conditions (S51);checking whether n-time fuel injection is needed or not (S52); setting afirst fuel injection amount (T₁) by correcting the basic fuel injectionamount (T_(p)) on the basis of various engine operating conditions (S53)when n-time fuel injection is not needed and further setting a firstfuel injection timing (T_(INJ1)) (S54); and setting a first fuelinjection amount (T_(i1)) when the n-time fuel injection is needed, asecond fuel injection amount (T_(i2)), a third fuel injection amount(T_(i3)), . . . (S55), in sequence; and further setting a firstinjection timing (T_(INJ1)), a second fuel injection timing (T_(INJ2)),a third fuel injection timing (T_(INJ3)), . . . (S56), in sequence.

Further, the third embodiment of the present invention furthercomprises, in addition to the steps of the first embodiment, an ignitiontiming retarding process including the steps of: detecting engineoperating conditions (N, T_(p)) (S201); checking whether exhaust gastemperature raising conditions are satisfied or not (S202); setting atarget exhaust gas temperature with reference to a map and on the basisof the detected engine operating conditions (N, T_(p)) (S203) when thetemperature raising conditions are satisfied; checking whether thetarget exhaust gas temperature is 300° C. or higher (S204); selectingtwice fuel injection and set the second fuel injection amount T_(i2)(S206) and the second fuel injection timing T_(INJ2) (S207) when thetemperature is 300° C. or higher; and selecting an ignition retard(S208) and set an ignition timing retard angle θ_(RD) (S209) when thetemperature is lower than 300° C.

Further, the fourth embodiment of the present invention furthercomprises, in addition to the steps of the third embodiment, a feedbackcontrol process including the steps of: checking whether exhaust gastemperature raising is executed or not (S301); measuring an exhaust gastemperature (S302) when the temperature raising is executed; checkingwhether the measured exhaust gas temperature reaches the target exhaustgas temperature (S303); checking whether the twice fuel injection or not(S304) when the gas temperature does not reach the target temperature;increase the second fuel injection amount (S305) when the gastemperature reaches the target temperature; and increase the ignitiontiming retard angle θ_(RD) (S306) when the gas temperature does notreach the target temperature.

Further, the fifth embodiment of the present invention furthercomprises, in addition to the steps of the first embodiment, an exhaustgas temperature raising process including the steps of: detecting engineoperating conditions (S401); detecting coolant temperature (S402);checking whether engine is being started (S403); set a target exhaustgas temperature (S404) when the engine is started; checking whether theset exhaust gas temperature is 400° C. or higher (S405); setting asecond injection fuel amount (T_(i2)) and a third injection fuel amount(T_(i3)) (S412) when the temperature is 400° C. or higher; and setting asecond injection fuel timing (T_(INJ2)) and a third injection fueltiming (T_(INJ3)) (S413).

Further, an exhaust gas temperature raising process further includes thesteps of: in the step of checking whether the engine is being started(S403) when the engine is not started, setting another target exhaustgas temperature (S414); checking whether thrice fuel injection isnecessary (S415); setting the thrice fuel injection amount (T_(i3)) andtiming (T_(INJ3)) (S412, S413) when the engine is started; selecting thetwice fuel injection (S407) when the engine is not started; and settingonly the second fuel injection amount (T_(i2)) and timing (T_(INJ2))(S408, S409).

Further, an exhaust gas temperature raising process further includes thesteps of: if no in the step of checking whether the target exhaust gastemperature is 400° C. or higher (S404); checking whether the targetexhaust gas temperature is 300° C. or higher; selecting the twice fuelinjection (S407) when the target temperature is 400° C. or higher; andsetting the second fuel injection amount (T_(i2)) and the second fuelinjection timing (T_(INJ2)) (S408, S409); selecting the ignition timingretard (S410) when the target temperature is not 400° C. or higher; andsetting the ignition timing retard rate (θ_(RD)) (S411).

Further, the sixth embodiment of the present invention furthercomprises, in addition to the steps of the first embodiment a combustionstatus detecting process including the steps of: detecting a pressure ofeach cylinder (S501); calculating heat generation rate (S502); checkingwhether the heat generation rate is normal or not (S503); executing thesecond fuel injection (S504) when the heat generation rate is normal;generating an alarm (S505) when the heat generation rate is not normaland interrupting the second fuel injection (S506); setting a first fuelinjection timing correction value ΔT_(M) (S507); setting a first fuelinjection amount correction value ΔK (S508); and setting an ignitiontiming correction value θ_(M) (S509).

Further, the seventh embodiment of the present invention furthercomprises, in addition to the steps of the first embodiment, acombustion status detecting process including the steps of: detecting apressure of each cylinder (S601); calculating heat generation rate(S602); checking whether a quantity of generated heat is large enough tofire the second fuel combustion or not (S603); executing the second fuelinjection (S604) when the quantity of generated heat is large enoughtherefor; selecting the first fuel combustion correction parameter(S605) when the quantity of generated heat is not large enough therefor;and setting a first fuel injection timing correction value (S607).

As described above, in the catalyst activation system for a direct fuelinjection engine according to the present invention, since the secondfuel is injected from the early period to the middle period of theexpansion stroke of the first (primary) combustion in such a way thatthe second injection fuel can be fired by the flame propagation of theprimary fuel, it is possible to fire the second injection fuel withoutdepending upon re-ignition (the possibility of mal-firing is large), sothat the exhaust gas temperature can be raised stably for catalystactivation.

Further, in the catalyst activation system according to the presentinvention, since any one of the twice fuel injection and the ignitiontiming retard method (for retarding the ignition timing of the primarycombustion) is selectively executed according to the engine operatingconditions, and furthermore since the thrice fuel injection after thesecond fuel injection is selectively executed according to theactivating condition of the catalyst, in such a way that the thirdinjection fuel can be fired by the flame propagation of the secondinjection fuel, it is possible to realize the early activation of thecatalyst, while keeping the fuel consumption rate at as low a level aspossible.

Further, in the catalyst control system according to the presentinvention, since the third or more additional fuels are injected intoeach engine cylinder, after the primary combustion, according to theengine operating conditions (e.g., as when an engine started at a lowtemperature) and the catalyst specifications (e.g., the target exhaustgas temperature is as high as 400° C. ), it is possible to activate thecatalyst more quickly and effectively, while keeping the fuelconsumption rate at as low as possible.

Further, in the catalyst activation control system according to thepresent invention, when a heat quantity large enough to fire the secondinjection fuel cannot be obtained by the primary combustion of the firstinjection fuel under the operating conditions which satisfy the twicefuel injection conditions, since the combustion status of the primarycombustion can be corrected (the heat quantity is increased), it ispossible to securely prevent raw gas from being emitted as it is, sothat the catalyst can be prevented from being overheated, degraded ordamaged due to a misfire of the second injection fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIG. 1 is an illustration for the combustion process in the single fuelinjection of the prior art direct fuel injection engine;

FIG. 2 is a timing chart for explaining the relationship between theheat generation pattern and NO_(x) generation rate, in the single fuelinjection of the prior art direct fuel injection engine;

FIG. 3 is an illustration for explaining the combustion process in thetwice fuel injections of the direct fuel injection engine according tothe present invention;

FIG. 4 is a timing chart for explaining the relationship between theheat generation pattern and NO_(x) generation rate, in the twice fuelinjections of the direct fuel injection engine according to the presentinvention;

FIG. 5 is a schematic block diagram showing an engine control system ofthe direct fuel injection engine according to the present invention;

FIG. 6 is a functional block diagram of an electronic control unitshowing a first embodiment of the catalyst activation control apparatusaccording to the present invention;

FIG. 7 is a graphical representation showing a twice fuel injectionrange, in which the engine speed N and the basic fuel amount T_(p) (anengine load) are taken as engine parameters;

FIG. 8 is a timing chart showing the relationship between the fuelignition timing and the first (primary) and second fuel injectiontimings corresponding to a crank angle after an engine top dead center;

FIG. 9A is a graphical representation showing the relationship betweenthe exhaust gas temperature and the second fuel injection timingcorresponding to the crank angle after top dead center (ATDC);

FIG. 9B is a graphical representation showing the relationship betweenthe combustion fluctuation rate and the second fuel injection timingnear ATDC;

FIG. 9C is a graphical representation showing the relationship betweenthe smoke concentration and the second fuel injection timing near ATDC;

FIG. 9D is a graphical representation showing the relationship betweenthe HC concentration and the second fuel injection timing near ATDC;

FIG. 9E is a graphical representation showing the relationship betweenthe fuel consumption rate and the second fuel injection timing nearATDC;

FIG. 10 is a graphical representation showing the relationship betweenthe fuel consumption rate and the exhaust gas temperature, in the secondfuel injection and the ignition timing retard;

FIG. 11 is a flowchart showing the procedure of the first fuel injectionsetting routine of the first embodiment;

FIG. 12 is a flowchart showing the procedure of the second fuelinjection setting routine of the first embodiment;

FIG. 13 is a functional block diagram of an electronic control unitshowing a second embodiment of the catalyst activation controlapparatus;

FIG. 14 is a flowchart showing the procedure of the n-time fuelinjection setting routine of the second embodiment;

FIG. 15 is a functional block diagram of an electronic control unitshowing a third embodiment of the catalyst activation control system;

FIG. 16 is a flowchart showing the procedure of a gas temperatureraising routine of the third embodiment;

FIG. 17A is a graphical representation showing the relationship betweenthe HC concentration and an indicated specific fuel consumption rate(ISFC), in the twice fuel injection, the ignition timing retard, and theintake throttle;

FIG. 17B is a graphical representation showing the relationship betweenthe exhaust gas temperature and the indicated specific fuel consumptionrate (ISFC), in the twice fuel injection, the ignition timing retard,and the intake throttle;

FIG. 18 is a functional block diagram of an electronic control unitshowing a fourth embodiment of the catalyst activation control system;

FIG. 19 is a flowchart showing the procedure of a gas temperatureraising routine of the fourth embodiment;

FIG. 20 is a functional block diagram of an electronic control unitshowing a fifth embodiment of the catalyst activation control system;

FIG. 21 is a flowchart showing the procedure of a gas temperatureraising routine of the fifth embodiment;

FIG. 22A is a graphical representation showing the relationship betweenthe HC concentration and the indicated specific fuel consumption rate(ISFC), in the twice fuel injection, the trice fuel injection, theignition timing retard, and the intake throttle;

FIG. 22B is a graphical representation showing the relationship betweenthe exhaust gas temperature and the indicated specific fuel consumptionrate (ISFC), in the twice fuel injection, the thrice fuel injection, theignition timing retard, and the throttle closing;

FIG. 23 is a functional block diagram of an electronic control unitshowing a sixth embodiment of the catalyst activation control system;

FIG. 24 is a flowchart showing the procedure of a first fuel injectionamount setting routine of the sixth embodiment;

FIG. 25 is a flowchart showing the procedure of a combustion statedetecting routine of the sixth embodiment;

FIG. 26 is a timing chart showing for assistance in explaining theabnormal combustion in the single fuel injection;

FIG. 27 is a functional block diagram of an electronic control unitshowing a seventh embodiment of the catalyst activation control system;and

FIG. 28 is a flowchart showing the procedure of a combustion statusdetecting routine of the seventh embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will become understoodfrom the following detailed description referring to the accompanyingdrawings.

(First Embodiment)

With reference to FIGS. 3 to 12, a first (basic) embodiment of thepresent invention will be explained hereinbelow.

The feature of the first embodiment is to inject a second injection fuelinto each engine cylinder from an early period to a middle period of anthe expansion stroke of the primary (first) fuel combustion in such away that the second injection fuel can be fired by the flame propagationof the primary fuel combustion, so that the exhaust gas temperature canbe raised securely for catalyst activation.

More in details, FIG. 3 shows the combustion process in the twice fuelinjection. As shown in FIG. 3, after a first injection fuel is injectedinto each engine cylinder, the first injection fuel is ignited for theprimary fuel combustion in the same way as with the case of theconventional method. After that, a second fuel is injected again. Here,inthe present invention, the secondly injected fuel is not re-ignited aswiththe case of the prior art apparatus, but fired by the flamepropagation of the first fuel combustion. By this method, it is possibleto eliminate thenon-combustion region (which cannot be fired by theprimary combustion of the first injection fuel) of the outermost mixtureof a lean air-fuel ratio. In addition, it is possible to further firethe gas remaining in the quench layer of the wall surface of thecombustion chamber and within crevices formed between a piston or pistonrings and the inner wall surface of the piston by the second combustionof the second injection fuel.

Further, FIG. 4 shows the relationship between the heat generationpattern and NO_(x) generation rate, in the twice fuel injection. Asshown in FIG. 4, it is possible to suppress the heat generated in thefirst half ofthe combustion, so that the generation of the NO_(x) can bereduced, thusimproving the combustion controllability. In other words,it is possible toimprove the combustion efficiency and to increase theexhaust gas temperature in the latter period of the combustion in theexpansion stoke of the engine for activating the catalyst.

In FIG. 5, in a direct fuel injection engine 1 according to the presentinvention, fuel is directly injected into each cylinder, and the mixtureof injected fuel and air is ignited by a spark plug 8. A throttle body 2having a throttle valve therein is attached to the engine 1, and an aircleaner 4 is connected to the upstream side of the throttle body 2 viaan air flow meter 3. On the other hand, a catalyst 5 for purifyingexhaust gas is disposed in an exhaust system of the engine 1, and amuffler 6 is attached to the downstream side of the catalyst 5.

Further, a fuel injector 7 each for injecting high pressure fueldirectly into each combustion chamber, and the spark plug 8 for ignitingthe injected fuel are both attached to the engine 1 so as to extend intoeach of the combustion chambers. Further, a cylinder pressure sensor 9for detecting an inner cylinder pressure is also attached to the engine1 so as to extend into the combustion chamber. Each fuel injector 6 andspark plug 8 are both arranged in a way that a relatively rich mixturecan be formed in the vicinity of the firing region of the spark plug 8by the fuel injected from the fuel injector 7; in other words, in such away suitable for stratified combustion.

Further, the spark plug 8 is connected to a secondary winding of anignition coil 10. An ignitor 11 is connected to a primary winding of theignition coil 10. The ignitor 11 is connected to an electronic controlunit (ECU) 20 for controlling the fuel injection and fuel ignition ofthe engine 1.

As shown in FIG. 5, the ECU 20 is a microcomputer for controlling theengine 1, which has a CPU 21, a ROM 22, a RAM 23, an input interface 34,an output interface 25, and a bus line 26.

To the input interface 24, there connected the air flow meter 3, thedirectpressure sensor 9, a throttle opening degree sensor 12 fordetecting the opening degree of the throttle valve in the throttle body2, a crank anglesensor 13 for detecting the crank angular position ofthe crankshaft, a cylinder discriminating sensor 14 for discriminating acylinder number, a coolant temperature sensor 15 for detecting thetemperature of the coolant, an exhaust gas temperature sensor 16 fordetecting exhaust gas temperature immediately before the catalyst 5.

On the other hand, the ignitor 11 is directly connected to the outputinterface 25. Further, a throttle actuator 17 and various actuators(e.g.,the fuel injectors 7) are connected via a driver circuit 29 or 30frespectively.

Fixed data, such as control programs, various maps, and etc. arepreviouslystored in the ROM 22. Various data obtained by processing thevarious sensor signals and data processed through various arithmeticsections of the CPU 21 are stored in the RAM 23. The CPU 21 calculatesvarious controlvariables (e.g., the amount of fuel injection, theinjection timing of eachof the cylinders of the engine 1) in accordancewith the control programs stored in the ROM 22 and on the basis of thevarious data stored in the RAM 23. The obtained control variables areoutputted respectively to the corresponding injector 7 and thecorresponding ignitor 11, etc. for fuel injection amount control andfuel injection timing control, so that the engine 1 can be maintained atoptimum combustion conditions.

In a normal engine combustion control, the fuel is injected and ignitedonce for each cycle; that is, the fuel is injected once from theinjector into the cylinder at the intake stroke or the compressionstroke, and after that the mixture of air and fuel is ignited by thespark plug 8. In the combustion control of the direct fuel injectionengine according to the present invention, under a predetermined engineoperating conditions, after the primary fuel injected into the cylinderin the suction or compression stroke has been ignited by the spark plug8, the subsidiary (or second) fuel is injected at least once into thecylinder in the expansion stroke of the primary fuel combustion, so thatthe outermost non-ignited gas can be fired by the flame propagation ofthe already-ignited primary combustion.

For instance, when the fuel is injected twice into the cylinder for eachcycle, the primary (first) fuel is first ignited by the spark plug 8.Further, at the latter period of the combustion in the expansion strokeofthe first fuel combustion, the subsidiary (second) fuel is injectedinto the cylinder again at such a timing as to be ignited by flamepropagation from the already-ignited portion of the preceding (thefirst) injection fuel, without re-ignition. In the same way, in the caseof the thrice fuelinjection, it is possible to further inject a thirdfuel at the latter period of the combustion in the expansion stroke ofthe second fuel combustion at such a timing as to be ignited by flamepropagation from thealready-fired portion of the preceding (the second)injection fuel, withoutre-ignition.

That is, as already explained with reference to FIG. 1, in the firstfuel injection, since the amount of the injected fuel is relativelyexcessive from the standpoint of fuel atomization, the fuel tends to bediffused at a wide angle, so that the outermost gas is excessively leanand thereby not ignited sufficiently. As a result, the amount ofexhausted hydrocarbon(HC) increases. Further, when the amount of thefuel injection is large, the heat generation rate tends to increase inthe first half of the combustion as shown in FIG. 2. This is because theinitial combustion becomes active. So that the gas ignited in the firsthalf is compressed inthe second half and thereby nitrogen oxide NO_(x)tends to be generated in the exhaust gas.

Accordingly, in the case where it is expected that the fuel is atomizedat a wide diffusion angle under the ordinary combustion control andthereby the outermost gas is excessively lean the amount of the primaryfuel injection to be ignited by the spark plug 8 is slightly reduced.Further, the second fuel is injected into the cylinder again at thelatter period of the expansion stroke of the primary fuel. Further, itis also possible to further inject a third, fourth, . . . fuel into thecylinder in the expansion stroke of the preceding injected fuel.

The function of the ECU 20 of the first embodiment will be described indetail hereinbelow with reference to FIG. 6. In the first embodiment ofthe present invention, in the case where the exhaust gas temperature islow (after the primary combustion by the ordinary (first) fuel injectionand ignition) and thereby the catalyst 5 cannot be activated, anadditional second fuel injection is achieved, immediately before theprimary combustion is completed, to generate the second combustion onthe basis of the flame propagation from the primary combustion (withoutdepending upon the second ignition), so that the exhaust gas temperaturecan be raised for activating the catalyst 5.

For achieving the above-mentioned function, the ECU 20 of the firstembodiment is provided with an additional control function for thesecond fuel injection, in addition to the ordinary first fuel controlfunction. That is, as shown in FIG. 6, the first (ordinary) fuel controlfunction can be realized by an engine speed calculate section 51, acrank position detect section 52, an operating condition detect section53, a first injection amount set section 54, a first injection timingset section 55, an ignition timing set section 56, a fuel injectiontiming calculate section 57, an ignition timing calculate section 58,and drive sections 59and 60. The second (invention) fuel controlfunction can be realized by a second injection condition discriminatingsection 61, a second injection amount set section 62, and a secondignition timing set section 63.

In more detail, the engine speed calculate section 51 calculates thenumberN of engine revolutions (referred to as an engine speed N,hereinafter) on the basis of the signals applied by the crank anglesensor 13. The crank angle detecting section 52 detects the crankangular position of each cylinder on the basis of the signal of thecrank angle sensor 13 and the signal of the cylinder discriminatingsensor 14.

The operating condition detecting section 53 first calculates a basicfuel amount T_(p) as an engine load (where T_(p) =K×Q/N; K: a correctionconstant determined for the injector 7; Q: an intake air amountobtainedby the signal of the air flow meter 3; and N: the engine speed obtainedby the engine speed calculate section 51), and then decides any one ofthe stratification combustion (for completing the fuel injectionimmediately before the ignition in the latter period of the compressionstroke) and the ordinary uniform combustion (for injecting fuel in theearly period of the intake stroke). That is, in the stratificationcombustion the rich mixture in the vicinity of the ignition region ofthe spark plug 8 is first ignited, and after that the lean mixture isfired bythe frame propagation (within the combustion chamber) of theignited rich mixture. On the other hand, in the uniform combustion, thefuel is ignitedafter the fuel is mixed with air uniformly within thecombustion chamber.

The first fuel injection amount set section 54 sets the fuel injectionamount (a first fuel injection amount T_(i1)) corresponding to theprimary (first) combustion according to the operating conditions. Inthis embodiment, the first fuel injection amount T_(i1) (T_(i1) =T_(p)×COEF+T_(s)) is calculated by correcting the basic fuel injection amountT_(p) obtained by the operating condition detecting section 53 onthebasis of a correction coefficient COEF for determined according tovarious engine operating conditions (parameters) and a voltagecorrection rate T_(s). The correction coefficient COEF is an incrementor decrementrate determined on the basis of the various engineparameters (engine operating conditions) obtained by the signals of thethrottle sensor 12, the coolant temperature sensor 15, etc. The voltagecorrection rate T_(s) is used to correct the response delay timing ofthe injector 7, because the actuation speed of the injector 7 issubjected to the influence of the battery voltage V_(B).

The first injection timing set section 55 sets the injection timing (thefirst injection timing T_(INJ1)) under the operating conditions of theengine speed N and the basic fuel injection amount T_(p) according tothe decided combustion method (uniform or stratification combustion)decided by the operating condition detecting section and outputs the setvalue to the fuel injection timing calculating section 57. This firstinjection timing T_(INJ1) is decided as an injection end timing T_(END)in the case of the stratified combustion and as the injection starttiming T_(ST) in the case of the uniform combustion.

The ignition timing set section 56 obtains a basic ignition timingθ_(BASE) decided on the basis of the engine speed N and the basic fuelinjection amount T_(p) calculated by the operating condition detectingsection 53, in accordance with a map retrieve, for instance. Further,the final ignition timing θ_(IG) is set by adding a coolant temperaturecorrecting lead angle value to the basic ignition timing θ_(BASE).

The fuel injection timing calculating section 57 calculates the firstinjection timing T_(INJ1) set by the first injection timing settingsection 55 in terms of the crank angle; that is, as the injection timingrelative to a reference crank angle position (e.g., a top dead center)of each cylinder (to be fuel-injected) detected by the crank angleposition detecting section 52, and outputs a signal corresponding to thefirst injection amount T_(i1) set by the first injection amount settingsection 54 to the injector 7 via the driver 59.

Further, when the second injection is executed, the fuel injectiontiming calculating section 57 calculates the second fuel injectiontiming T_(INJ2) set by the second injection timing setting section 63 interms of the crank angle; that is, as the injection timing relative to areference crank angle position of each cylinder (to be fuel-injected)detected by the crank angle position detecting section 52, and outputs asignal corresponding to the second fuel injection amount T_(i2) set bythe second injection amount setting section 62 (described later) to theinjector 7 again via the driver 59.

The ignition timing calculating section 58 calculates the ignitiontiming θ_(IG) set by the ignition timing setting section 56 in terms ofthe crank angle; that is, as the ignition timing relative to a referencecrank angle position of each cylinder (to be fuel-ignited) detected bythecrank angle position detect section 52, and outputs a signalcorresponding to the ignitor 11 via the driver 60.

On the other hand, the second injection condition discriminating section61discriminates whether the current operating range lies within a rangewhichrequires the second fuel injection on the basis of the engine speedN and the basic fuel injection amount T_(p) detected by the operatingcondition detecting section 53. If the second fuel injection isdeterminedto be necessary, the second injection condition discriminatingsection 61 outputs a second fuel injection command to the secondinjection amount setting section 62 and the second injection timingsetting section 63.

In practice, the second fuel injection range can be discriminated byusing a map (as shown in FIG. 7) stored in the ROM 22. This mapindicates a second fuel injection range (hatched portion) decidedempirically, in which the abscissa designates the engine speed N and theordinate designates the basic fuel injection amount T_(p) (engine load)detected by the operating condition detect section 53. In this secondfuel injection range, the exhaust gas temperature is low because thefirst fuelinjection amount for the primary combustion is small due to alow engine speed N and a low engine load in the stratificationcombustion, and thereby it is difficult to activate the catalyst 5. Insummary, by retrieving from the map in the ROM 22, the second injectioncondition discriminate section 61 discriminates whether the currentengine operatingconditions satisfy the conditions designated by thesecond fuel injection range as shown in FIG. 7, and outputs a commandwhen the second fuel injection is necessary.

In response to this command outputted by the second injectiondiscriminating section 61, the second injection amount setting section62 sets the second injection amount T_(i2) fired by flame propagation ofthe primary combustion by the first injection. In this embodiment, thesecond injection amount T_(i2) is decided on the basis of the basicfuelinjection amount T_(p) (the basic value of the first injectionamount T_(i1)). Without being limited thereto, however, it is alsopossible to decide a minimum fuel injection amount fired by the flamepropagation of the primary combustion by some experiments and further toadjust the minimum amount in the incremental direction.

The second injection timing setting section 63 sets the second injectiontiming (at which the second injection fuel can be securely fired by theflame propagation of the primary combustion) according to the operatingconditions determined on the basis of the engine revolution speed N andthe basic fuel injection amount T_(p), by a map retrieval, for instance.

More in details, it is necessary to decide the second fuel injectiontimingat an appropriate range in order to fire the second injection fuelin the latter period (i.e., the expansion stroke) of the primarycombustion without reignition. For instance, FIG. 8 shows therelationship between the fuel ignition timing and the first (primary)and second fuel injectiontiming, in relation to the crank angle after atop dead center. As shown inFIG. 8, when the fuel injection start timingis determined roughly between 10 and 80 degrees after the top deadcenter, the stable firing of the second injection fuel can be obtainedwithout re-ignition. Further, when the fuel injection start timing isdetermined roughly between 30 and 60 degrees after the top dead center,the optimum firing of the second injection fuel can be obtained withoutreignition. However, smoke is produced, when the second injection timingis determined within an excessively short time after the primarycombustion timing. This is because the fuel cannot be sufficientlydiffused. On the other hand, not only the exhaust gas temperature cannotrise high, but also HC increases when the second injection timing isdetermined within an excessively long time after the primary combustiontiming. This is because the fuel is diffused excessively. So that, theexhaust emission deteriorates.

Accordingly, it is necessary to determine the optimum injection timingof the second injection fuel on the basis of some experiments bydetermining the engine speed N and the basic fuel injection amount T_(p)(engine load) as the parameters and to store the experiment results asspecified map. The second injection timing set section 63 can set anappropriate second fuel injection timing T_(INJ2) according to theoperating conditions by retrieving from the map. Although the optimumvalue of the second injection timing T_(INJ2) differs according to theengine specification, the number of cycles (e.g., 2 cycles, 4 cycles,etc. ), etc., it is possible to use the same optimum injection timingfor all the direct fuel injection engine qualitatively, so that there-ignition is notrequired for firing the second injection fuel. In theengine 1 of the present invention, the second fuel injection timing isdetermined between the early period and the middle period of theexpansion stroke of the primary combustion; that is, between 30 and 60degrees after the top dead center (ATDC).

In more detail, FIGS. 9A to 9E show the experiment results, whichindicate that the optimum second injection start timing after top deadcenter (ATDC) lies between 30 and 60 degrees. Further, FIG. 9A shows therelationship between the exhaust gas temperature and the second fuelstarttiming in relation to the crank angle after top dead center (ATDC);FIG. 9Bshows the relationship between the fluctuation rate of thecombustion and the same start timing in relation to ATDC; FIG. 9C showsthe relationship between the concentration of smoke and the same starttiming in relation to ATDC; FIG. 9D shows the relationship between theconcentration of HC and the same start timing in relation to ATDC; andFIG. 9E shows the relationship between the fuel consumption rate and thesame start timing in relation to ATDC.

As explained above, it is necessary to raise the exhaust gas temperaturetosome extent in order to maintain the activation of the catalyst. Inthe conventional method, when the exhaust gas temperature is excessivelylow after normal primary fuel injection, the second fuel injection orthe second fuel ignition timing has been retarded to shift the secondcombustion phase, as shown by dashed-line in FIG. 10 which shows therelationship between the fuel consumption rate and the exhaust gastemperature. In this prior art method, however there exists a problem inthat the exhaust gas temperature still cannot be raised sufficientlyhigh or the fuel consumption rate deteriorates remarkably.

In the first embodiment, since the second fuel is additionally injectedandfired by the flame propagation of the already-burnt fuel (withoutre-ignition), it is possible to improve the fuel combustioncontrollability after the first fuel is injected and further ignitedonce for each cycle, so that the exhaust gas temperature can be raisedeffectively and securely for activation of catalyst, as shown by a solidline in FIG. 10, without generating HC and NO_(x), in an excellent fuelconsumption rate.

Further, in the above-mentioned first embodiment, although the secondfuel injection amount and the timing are both decided on the basis ofthe engine speed N and the engine load (i.e., the basic fuel injectionamount T_(p) for the primary fuel combustion), without being limitedonly thereto, it is also possible to adopt the other engine operatingconditions such as the intake air amount per cycle, the pressure inintakepipe, etc. as far as the same effect can be obtained.

The operation of the second fuel injection controlled by the ECU 20shown in FIG. 6 will be described hereinbelow with reference to theflowchart shown in FIG. 11 (the first fuel injection setting routine)and FIG. 12 (the second fuel injection setting routine).

First, in the first fuel injection setting routine shown in FIG. 11, instep S101, the CPU 21 (referred to as control simply, hereinafter) readsthe basic fuel injection amount T_(p) on the basis of a predeterminedaddress of the RAM 23. In step S102, the control reads the engineoperating condition parameters such as the signals from the throttlesensor 12f the coolant temperature sensor 15, and etc. In this case, theorder of step S101 and step S102 can be exchanged with each other.

Further, in step S103, the control sets a coefficient COEF forcorrecting the basic fuel injection amount, which is determinedaccording to various engine operating conditions (e.g., to increase ordecrease the fuel amount) by reading in the step S102. In step S104, thecontrol further sets the voltage correction rate T_(s) on the basis of abattery voltageV_(B), proceeding to step S105.

In step S105, the control sets the first injection amount T_(i1) (T_(i1)←T_(p) ×COEF+T_(s)) by multiplying the basic fuelinjection amount T_(p)read in step S101 by the correction coefficient COEF determinedaccording to the engine operation conditions and set in step S103 andfurther by adding the voltage correction rate T_(s) set instep S104.Further, in step S106, the control sets the first fuel injectiontimingT_(INJ1) according to the combustion method (uniform or stratificationcombustion), passing through the routine.

In the second fuel injection setting routine shown in FIG. 12, in stepS151, the control detects the engine operating conditions by reading theengine speed N and the engine load (the basic fuel injection amountT_(p)). In step S152, the control checks whether the current operatingconditions satisfy the second fuel injection conditions. As explainedabove with reference to FIG. 7, the second injection conditions can bedecided by the map; more in details, by checking whether or not theenginespeed N and the basic fuel injection amount T_(p) (engine load)lie within the second fuel injection range (the hatched-line portion).

As a result, when the second fuel injection conditions are notsatisfied, for instance as when the engine is driven under a high loadin the uniformcombustion or when driven at a high speed but under a lowload in the stratified combustion the control passes through theroutine, without executing the second fuel injection. However, when thesecond fuel injection conditions are satisfied, for instance, as whenthe engine is driven at a low speed and under a low load in thestratified combustion, the control proceeds to step S153 to set thesecond fuel injection amount T_(i2). Further, in step S154, the controlsets an optimum second fuel injection timing T_(INJ2) by the map on thebasis of the engine speed N and the basic fuel injection amount T_(p)(engine load), passing throughthe routine.

Upon ending the above-mentioned respective routines, the setted firstinjection amount T_(i1) (T_(i1) ←T_(p) ×COEF+T_(s)) andthe first fuelinjection timing T_(INJ1) are set in a fuel injection timer and a fuelignition, respectively. The fuel injection timer starts when thecylinder to be injected reaches a predetermined reference crank angularposition (e.g., top dead center), to inject the first fuel into thecylinder. In the same way, the fuel ignition timer starts when thecylinder to be ignited reaches a predetermined reference crank angle, toignite the first injection fuel for the primary combustion start.

Further, when the second fuel injection conditions are satisfied in thestratified combustion (which is low fuel consumption rate and betterexhaust gas character), the first fuel injection amount T_(i1) isinjected in the latter period of the compression stroke, and the secondinjection amount T_(i2) is injected into the cylinder at such anoptimumtiming immediately before the end of the primary combustion bythe first injection, to start the second combustion by the flamepropagation of the primary combustion, without need of re-ignition.Here, the second fuel combustion is mainly used to increase only theexhaust gas temperature, without moving the piston.

In other words, since the second injection fuel is not fired by there-ignition but fired by the flame propagation from the primary fuelcombustion, it is unnecessary to ignite fuel twice in each cycle, sothat the second combustion can be realized securely, while economizingthe ignition energy. In addition, since the second injection fuel isinjected at an optimum injection timing, it is possible to increase theexhaust gastemperature stably for activation of the catalyst, withoutgenerating smokeand increasing HC, thus keeping the fuel consumptionrate at a minimum level.

(Second Embodiment)

A second embodiment will be described hereinbelow with reference toFIGS. 13 and 14, which is different from the first embodiment in thatthe additional fuel injection and ignition by the flame propagation ofthe preceding fuel combustion are executed N-timings according to theengine operating conditions.

In FIG. 13, the ECU 20 further comprises an exhaust gas temperaturesensor 16. Further, the first and second fuel injection amount settingsections 54 and 62 of the first embodiment shown in FIG. 6 are combinedas a singlefuel injection amount set section 54A, and further the firstand second fuel ignition timing setting sections 55 and 63 of the firstembodiment shown in FIG. 6 are combined as a single fuel ignition timingsetting section 55A. Further, the second injection conditiondiscriminating section 61 shown in FIG. 6 is replaced with anotherinjection condition discriminating section 61A for executing N-time fuelcombustion.

More in details, the injection discriminating section 61A discriminateswhether a plurality of the additional fuel injections are necessary ornotunder the current engine operating conditions, and outputs aplurality of commands to the fuel injection amount setting section 54Aand the fuel injection timing setting section 55A according to thediscrimination results.

Further, the second and after injection ranges can be discriminated byusing a plurality of maps (as shown in FIG. 7) stored in the ROM 22,repeatedly. These maps indicate a second injection range (hatchedportion)decided empirically, in which the abscissa designates the enginespeed N and the ordinate designates the engine load (basic fuelinjection amount T_(p) detected by the operating condition detectingsection 53. In this second injection range, the exhaust gas temperatureis low after the primary combustion of the first fuel injection, andthereby it is difficult to activate the catalyst 5, as when the lowengine speed and theengine load are both low in the stratifiedcombustion). By retrieving from the map in the ROM 22, the injectioncondition discriminating section 61A discriminates whether the currentengine operating range satisfies the second fuel injection range or not.

Further, after the second fuel injection, the injection conditiondiscriminating section 61A further discriminates whether the third fuelinjection is necessary or not to maintain the active status of thecatalyst 5 on the basis of the exhaust gas temperature detected by theexhaust gas temperature sensor 16. Further, after the third fuelinjection, the fourth, fifth, . . . fuel injection conditions arediscriminated by the injection condition discriminating section 61A.

The injection amount setting section 54A sets the fuel injection amountaccording to the number of fuel injections per cycle designated by theinjection condition discriminating section 61A. That is, when theinjection discriminating section 61A designates one fuel injection percycle the fuel injection amount T_(i) corresponding to the first(primary) fuel combustion is set according to the engine operatingconditions. This fuel injection amount T_(i) can be set by correcting(increasing or decreasing) the basic fuel injection amount T_(p)(calculated by the operating condition detecting section 53) on thebasis of the correction coefficient COEF determined according to variousengine operating conditions decided by the throttle sensor 12, thecoolant temperature sensor 15, etc. ) and the voltage correction rateT_(s) (forcorrecting the response delay timing of the injector 7 due tochange in a battery voltage V_(B)).

Further, when the injection discriminating section 61A designates two ormore fuel injections per cycle, the fuel injection amount settingsection 54A sets the first fuel injection amount T_(i1) slightly smallerthan the basic fuel injection amount T_(i) (to be ignited by the sparkignition), and further the second and after fuel injection amountsT_(i2) T_(i3), . . . (to be fired by the flame propagation of thepreceding fuel combustion).

The first fuel injection amount T_(i1) can be set with reference to amapobtained empirically, in which a fuel injection amount for providingan optimum fuel atomization diffusion and an optimum heat generationrate is determined according to the engine operating conditions such asthe enginespeed N and the engine load (i.e., the basic fuel injectionamount T_(p)). Further, the second and after fuel injection amountsT_(i2), T_(i3), . . . , are set in such a way as to be fired by theflame propagation of the preceding combustion, in sequence at apredetermined ratio to the first fuel injection amount T_(i1), forinstance. Further, the third and after fuel injection rates T_(i3),T_(i4), . . . are determined to be equal to or slightly larger than thesecond fuel injection amount T_(i2).

In the case of the normal (single) fuel injection per cycle, the fuelinjection timing set section 55A decides a fuel injection timingT_(INJ1) on the basis of the combustion (uniform or stratification)method decided by the operating condition detecting section 53, inresponse to the command applied from the injection conditiondiscriminating section 54, and under the engine operating conditionsdetermined by the engine speed N and the basic fuel injection amountT_(p). In the same way, in the case of the n-time fuel injections percycle, the fuel injection timing set section 55A decides a first fuelinjection timing T_(INJ1), a second fuel injection timing T_(INJ2), athird fuel injection timing T_(INJ3), . . . , and outputs the decidedfuel injection timings to the fuel injection timing calculating section58, in sequence.

In this case, the primary injection timing T_(INJ1) and the firstadditional fuel injection timing T_(INJ1) are both decided as theinjection start timings in the case of the uniform combustion method,and as the injection end timing in the case of the stratified combustionmethod. Further, the second and after fuel injection timings T_(INJ2),T_(INJ3), . . . are decided with reference to a map determinedaccordingto the engine operating conditions (the engine speed N and thebasic fuel injection amount T_(p)), for instance in such a way that then-th injected fuel can be securely fired by the flame propagation of thepreceding combustion.

For instance, when the fuel injections are executed twice, for instance,inorder to securely fire the second injection fuel without re-ignition,it ispreferable that the second injection timing is determined roughlybetween 10 and 80 degrees after the top dead center as shown in FIG. 8.However, when the second injection timing is determined within anexcessively shorttime immediately after the primary combustion timing,since the fuel cannotbe diffused sufficiently, smoke is produced due tolack of diffusion. On the other hand, since the fuel is diffusedexcessively when the second injection timing is determined within anexcessively long time after the primary combustion timing, not only theexhaust gas temperature cannot rise high, but also HC increases, so thatthe exhaust emission deteriorates.

Accordingly, it is necessary to determine the optimum injection timingson the basis of some experiments by setting the engine speed N and theengineload (the basic fuel injection amount T_(p)) as the parameters andto store the experiment results as specified maps. The injection timingsetting section 55A can set appropriate second and after injectiontimingsT_(INJ2), T_(INJ3), . . . according to the operating conditionsin accordance with the map retrieval. Although the optimum values of thethese injection timings T_(INJ2), T_(INJ3), . . . differ according tothe engine configuration and the number of cycles (e.g., 2 cycles, 4cycles, etc. ), it is possible to use the same optimum injection timingsfor all the direct fuel injection engine qualitatively, so that there-ignitions are not required for the second and after fuel injection.

In the engine 1 of the present invention, the second fuel injection isdetermined empirically between the early period and the middle period ofthe expansion stroke of the primary combustion. In more practice, asshownin FIG. 8, it is preferable to set the optimum injection timingbetween 30 and 60 degrees after the top dead center (ATDC). Further, thethird injection fuel is injected into the cylinder at such a timingimmediately before the second fuel injection ends and thereby the thirdinjection fuelcan be fired by the flame propagation of the second fuelcombustion. In more practice, the third fuel injection is set roughlybetween 90 and 120 degrees after the top dead center. Further, each ofthe fourth and after fuel injection timings is set at such a timing asto be injected into the cylinder immediately before the preceding fuelinjection ends in the expansion stroke and thereby the injected fuel canbe fired by the flame propagation of the preceding fuel combustion.

The ignition timing setting section 56 obtains a basic ignition timingθ_(BASE) decided on the basis of the engine speed N and the basic fuelinjection amount T_(p) calculated by the operating condition detectingsection 53, in accordance with a map retrieve, for instance. Further,the final ignition timing θ_(IG) is set by adding a coolant temperaturecorrecting lead angle value to the basic ignition timing θ_(BASE).

The fuel injection timing calculating section 57 calculates the firstinjection timing T_(INJ1) set by the first injection timing settingsection 55A in terms of the crank angle relative to the reference crankangle position of each of the cylinder (to be fuel-injected) detected bythe crank angle position detecting section 52, and outputs signalscorresponding to the injection amounts set by the injection amountsettingsection 54A and the injection timings set by the injection timingsetting section 55A to the injector 7 via the driver 59.

The ignition timing calculating section 58 calculates the ignitiontiming θ_(IG) set by the ignition timing setting section 56 in terms ofthe crank angle relative to the reference crank angle position of eachcylinder (to be fuel-ignited) detected by the crank angle positiondetecting section 52, and outputs a signal corresponding thereto to theinjector 11 via the driver 60.

The fuel injection setting process executed by the ECU 20 will bedescribedhereinbelow with reference to the flowchart shown in FIG. 14.

The routine as shown in FIG. 14 is executed for each predeterminedperiod. In step S51, the control reads the engine operating conditions(i.e., the engine speed N and the basic fuel injection amount T_(p)) andthe numberof fuel injections per cycle. In step S52, the control checkswhether the n-time fuel injection conditions are satisfied on the basisof the currentengine operating conditions. In this step S52, if thepreceding fuel injection is the normal (single) fuel injection, thecontrol checks whether the current operating conditions lie within thesecond fuel injection range on the basis of the engine speed N and thebasic fuel injection amount T_(p) (engine parameters). If the secondfuel injectionhas been already executed at the preceding fuel injection,control checks whether the third and after fuel injections are necessaryon the basis of the exhaust gas temperature detected by the exhaust gastemperature sensor

As a result, when the second or after fuel injections are not requiredas when the engine 1 is operating under a high load in the uniformcombustionor when the engine 1 is operating at a high speed and under alow load evenin the stratified combustion, the control proceeds fromstep S52 to step S53 to execute the ordinary (single) fuel injection percycle. On the other hand, when the second and after fuel injections areneeded as with the case where the engine 1 is operating at a low speedand under a low load in the stratified combustion, the control proceedsto step S55 to execute the n-time fuel injections per cycle.

In step S53 (single fuel injection per cycle), the control sets thefirst injection amount T_(i1) (T_(i1) ←T_(p) ×COEF +T_(s)) bycorrectingthe basic fuel injection amount T_(p) on the basis of the correctioncoefficient COEF (to increase the fuel amount) determined by the engineoperating parameters represented by the signals of the throttlesensor12, the coolant temperature sensor 15, etc. and the voltage correctionrate T_(s) determined on the basis of the battery voltage V_(B).Further, in step S54, the control sets the fuel injection timing T_(INJ)according to the combustion method, passing through the routine.

When the second and after fuel injections are needed per cycle, in stepS55, the control sets the first (primary) fuel injection amount T_(i1)in which the optimum fuel atomization diffusion and heat generation canbeobtained, with reference to the map. Further, control sets the secondfuel injection amount T_(i2) and the third additional fuel injectionamount T_(i3), . . . by multiplying the first fuel injection amountT_(i1) bya predetermined ratio, respectively so that the injected fuelcan be fired by the flame propagation of the preceding combustion.

Further, in step S56, in the same way as the case of the injectiontiming T_(INJ) in the ordinary uniform combustion, the control sets thefirst fuel injection timing T_(INJ1) and further the second fuelinjection timing T_(INJ2), the third fuel injection timing T_(INJ3), . .. with reference to the maps determined on the basis of the engine speedN and the basic fuel injection amount T_(p), so that the second andafter injection fuels can be securely fired by the flame propagation ofthe preceding combustion, passing through the routine.

Upon ending the above-mentioned routine, the first fuel injection amountand the injection timing setted in the steps S53 and S55 are both set toafuel injection timer and a fuel ignition timer, respectively. Further,the fuel injection timer is started from a predetermined reference crankangleposition of the cylinder, to inject the first fuel into thecylinder. In the same way, the fuel ignition timer is started from apredetermined reference crank angle position of each cylinder, to ignitethe first fuel,thus the mixture in the combustion chamber is ignited forthe primary combustion for each cylinder.

In this case, when the second and after fuel injections are executedunder the stratified combustion (which is low in the fuel consumptionand betterexhaust gas characteristics), the first (primary) fuelinjection amount T_(i1) is injected at the latter period of thecompression stroke, and the second injection amount T_(i2) is injectedinto the cylinder at suchan optimum timing immediately before theprimary combustion by the first fuel injection ends, to start the secondcombustion by the flame propagation of the primary combustion withoutneed of re-ignition. Here, the second fuel combustion is mainly consumedto increase only the exhaustgas temperature, without moving the piston.However, when the exhaust gas temperature does not rise sufficientlyhigh after the second fuel combustion, the third fuel injection amountT_(i3) is injected at the same expansion stroke to continue there-combustion due to the flame propagation of the preceding combustion.

As already explained, in order to maintain the activation of thecatalyst 5, it is necessary to raise the exhaust gas temperature to someextent. Inthe conventional method, when the exhaust gas temperature isexcessively low after the ordinary primary fuel injection, the fuelinjection or the ignition timing has been retarded to shift thecombustion phase, as shown by dashed line in FIG. 10. In the prior artmethod, however, there exists a problem in that the exhaust gastemperature still cannot be raised sufficiently high or the fuelconsumption rate deteriorates remarkably.

In the second embodiment, however, after the first fuel is injected andignited once for each cycle, since the second and after fuels areadditionally injected and ignited by the flame propagation of thepreceding combustion, it is possible to increase the fuel combustioncontrollability, so that the exhaust gas temperature can be raisedeffectively for activation of catalyst, as shown by a solid line in FIG.10, without generating HC and NO_(x) in an excellent fuel consumptionrate.

In this case of the second embodiment, since the second and after fuelinjection are not ignited by the spark plug 8 but fired by the flamepropagation from the preceding combustion, it is unnecessary to ignitefuel twice or more in each cycle, so that the second and aftercombustion can be realized securely, while economizing the ignitionenergy. In addition, since the second and after fuels are injected at anoptimum injection timing, respectively, it is possible to increase theexhaust gastemperature stably for activation of catalyst 5, withoutgenerating smoke and increasing HC, while keeping the fuel consumptionat a minimum level.

(Third Embodiment)

The third embodiment will be described hereinbelow with reference toFIGS. 15, 16 and 17B and 17B. This third embodiment is different fromthe first embodiment shown in FIG. 6 in that any one of the twice fuelinjection method and the ignition timing retard method (the primary fuelignition timing is retarded to increase the exhaust gas temperature) isselected according to the engine operating conditions. In FIG. 15, atarget exhaustgas temperature setting section 71 is provided instead ofthe second injection condition discriminating section 61, and further atemperature raising method selecting section 72 and an ignition timingretard section 73 are additionally provided, in addition to the elementsshown in FIG. 6.

The target exhaust gas temperature setting section 71 discriminateswhetherthe current engine operating conditions lie within an operatingrange in which the catalyst 5 cannot be activated, that is, in anexhaust gas temperature raising range or not. If the exhaust gastemperature raising range is determined, this target exhaust gastemperature setting section 71 sets a target temperature of the exhaustgas. The setted target exhaustgas temperature is previously stored in amap, for instance under consideration of the characteristics of thecatalyst 5. This map is retrieved by using the engine speed N and theengine load (the basic fuel injection amount T_(p)) as parameters, andthe retrieved value is outputted to the temperature raising methodselecting section 72.

The temperature raising method selecting section 72 selects any one ofthe method of raising the exhaust gas temperature by injecting fueltwice and the method of raising the same by retarding the ignitiontiming of the primary combustion, according to the target exhaust gastemperature settedby the target exhaust gas temperature setting section71. FIG. 17A shows the relationship between the indicated specific fuelconsumption rate ISFC(the abscissa) and the HC concentration (theordinate) and FIG. 17B shows the relationship between the indicatedspecific fuel consumption rate ISFC(the abscissa) and the exhaust gastemperature (the ordinate) both in idling operation, in which the solidlines designate the method of retarding the ignition timing of theprimary combustion; the alternate long and short lines designate themethod of injecting fuels twice; and the alternate long and two shortdashes lines designate the method of throttling the intake pipe. FIG.17B indicates that the twice fuel injection is effective as the effectof raising the exhaust gas temperature, as compared with the ignitiontiming retarding method. However, the fuel consumption rate increases inthe case of the twice fuelinjection.

Therefore, in order to realize the maximum temperature raising effect inthe minimum possible fuel consumption rate, it is preferable to changetheexhaust gas temperature raising method according to the targetexhaust gas temperature. That is, when the target exhaust gastemperature is relatively low, the ignition timing retarding method isselected to economize the fuel. On the other hand, when the targetexhaust gas temperature is relatively high, the twice fuel injectionmethod is selected to raise the exhaust gas temperature quickly.

In this third embodiment, when the target exhaust gas temperature isequal to or slightly higher than 300° C., the ignition timing retardingmethod is selected, so that the temperature raising method selectingsection 72 outputs an ignition timing retarding command. On the otherhand, when the target exhaust gas temperature is higher than 300° C.,the twice fuel injection method is selected, so that the temperatureraising method selecting section 72 outputs a twice fuel injectioncommand, as shown in FIG. 17B.

Therefore, when the temperature raising method selecting section 72selectsthe twice fuel injection method to raise the exhaust gastemperature, as already explained in the first embodiment, the secondinjection amount setting section 62 sets the second fuel injectionamount T_(i2), and thesecond injection timing setting section 62 set thesecond fuel injection timing T_(INJ2).

Further, when the temperature raising method selecting section 72selects the ignition timing retarding method to raise the exhaust gastemperature,the ignition timing retard section 73 sets an ignitiontiming retarding rate θ_(RD) by retrieving an ignition timing retardingangle correction map decided on the basis of the engine speed N and thebasic fuel injection amount T_(p). The set ignition timing retardingrate θ_(RD) is outputted to the ignition timing setting section 56.

In this third embodiment, after the first (primary) fuel injectionsetting routine (as explained in the first embodiment with reference toFIG. 11) has been executed, the exhaust gas temperature raising routineas shown inFIG. 16 is executed. That is, after the first (primary) fuelinjection, anyone of the twice fuel injection and the ignition timingretarding is selected to raise the exhaust gas temperature.

In the exhaust gas temperature raising routine shown in FIG. 16, in stepS201, the control detects the operating conditions on the basis of theengine speed N and the engine load (the basic fuel injection amountT_(p)). In step S202, the control discriminates whether the exhaust gastemperature raising conditions are satisfied. If no, the control passesthrough the routine. If yes, the control proceeds to step S203 toexecute the map retrieval on the basis of the engine speed N and thebasic fuel injection amount T_(p), and further sets a target exhaust gastemperature according to the engine operating conditions.

In step S204, the control checks whether the target exhaust gastemperatureset in step S203 is higher than 300° C. If yes, in step S208,the control selects the twice fuel injection method. If no, in stepS208, the control selects the ignition retarding method.

When the twice fuel injection method is selected in step S205, thecontrol sets the second fuel injection amount T_(i2) and the second fuelignition timing T_(INJ2) in steps S206 and S207, respectively, passingthrough the routine. Further, when the ignition retard is selected instepS208, the control sets an ignition timing retard angle θ_(RD) instep S209, passing through the routine.

In this third embodiment, since the optimum processing for activatingthe catalyst, that is, for increasing the exhaust gas temperature can beselected according to the engine operating conditions to save fuel, itis possible to obtain the maximum exhaust gas temperature rasing effectin the minimum fuel consumption rate. Accordingly, an increase of thefuel consumption can be kept at the minimum value. Further, in thisthird embodiment, although an example of engine idling operation hasbeen explained, the same processing as described above can be executedin the same way by setting the target exhaust gas temperature on thebasis of therespective engine operating conditions (engine speed andload) other than the engine idling operation.

(Fourth Embodiment)

The fourth embodiment will be described hereinbelow with reference toFIGS.18 and 19. This fourth embodiment is different from the thirdembodiment shown in FIG. 15 in that the signal of the exhaust gastemperature sensor 16 is feed-backed to correct the second fuelinjection amount T_(i2) andthe ignition timing retard angle θ_(RD), sothat the exhaust gas temperature can be raised to the target exhaust gastemperature under the feedback control.

In FIG. 18, therefore, the ECU 20 further comprises a compare section74, and further the exhaust gas temperature sensor 16 is connected tothis compare section 16.

The comparing section 74 compares the actual exhaust gas temperaturedetected by the exhaust gas temperature sensor 16 with the targetexhaust temperature set by the target exhaust gas temperature settingsection 71, and outputs an output signal indicative of the differencebetween the actual and target exhaust gas temperatures. On the basis ofthe output signal of the comparing section 74, the second injectionamount T_(i2) is adjusted by the second injection amount setting section62, and the ignition timing correction angle θ_(RD) is adjusted by theignition timing retard section 73, respectively.

In this fourth embodiment, after the exhaust gas raising routine asexplained in the third embodiment (shown in FIG. 16) has been executed,the exhaust gas temperature feedback routine as shown in FIG. 19 isexecuted to control the actual exhaust gas temperature to the targetvaluethereof within a feedback loop.

With reference to FIG. 19, in step S301, the control checks whether theexhaust gas temperature raising is executed or not. If not executed, thecontrol passes through the routine. If executed, the control proceeds tostep S302 to measure the actual exhaust gas temperature on the basis ofthe signal of the exhaust gas temperature sensor 16.

Further, in step S303, the control checks whether the actual exhaust gastemperature measured in step S302 reaches the target exhaust gastemperature. If reaches the target exhaust gas temperature, the controlpasses through the routine. If not reaches the target value, in stepS304,the control checks whether the selected exhaust gas raising methodis the twice fuel injection or not.

If the twice fuel injection method is selected, in step S305, thecontrol increases the second fuel injection amount T_(i2), passingthrough the routine. If the ignition timing retard method is selected,in step S306, the control increases the ignition timing retard angleθ_(RD), passing through the routine.

As described above, in this fourth embodiment, it is possible toconverge the exhaust gas temperature to the target value quickly, sothat the exhaust gas temperature can be raised effectively at a highspeed for activating the catalyst 5.

(Fifth Embodiment)

The fifth embodiment will be described hereinbelow with reference toFIGS. 20 and 21 and FIG. 22A and 22B. This fifth embodiment is differentfrom the third embodiment shown in FIG. 15 in that after the second fuelinjection, a third fuel injection is further executed according to theactivation conditions of the catalyst.

In FIG. 20, therefore, the ECU 20 further comprises a catalystactivation discriminating section 75, a third injection amount settingsection 76, a third injection timing setting section 77, in addition tothe third embodiment shown in FIG. 15. Further, the functions of thetarget exhaust gas temperature setting section 71 and the temperatureraising method selecting section 72 are slightly modified.

The catalyst activation discriminating section 75 discriminates whetherthecatalyst reaches the activation temperature or not on the basis ofthe signal of the exhaust gas temperature sensor 16, and outputs thediscriminated result to the temperature rasing method selecting section72.

On the other hand, the target exhaust gas temperature setting section 71sets the target exhaust gas temperature according to the current engineoperating conditions (e.g., the engine starting operation, the steadystate operation, etc. ). This target exhaust gas temperature is usuallyset to 400° C. or higher for early activation of the catalyst, whentheengine 1 is started at a low temperature. On the other hand, when theengine 1 is not started but operated stably, as already explained in thefourth embodiment, the target exhaust gas temperature is set to a valuelower than 400° C., which is stored as a map by setting the engine speedN and the basic fuel injection amount T_(p) as parameters inconsideration of the characteristics of the catalyst 5.

The temperature raising method selectting section 72 selects any one ofthethrice fuel injection method, twice fuel injection method and theignition timing retard method, to raise the exhaust gas temperature,according to the target exhaust gas temperature set by the targetexhaust gas setting section 71. In this fifth embodiment, when thetarget exhaust gas temperature is set to a value 400° C. or higher aswhen the engine is started at a low temperature (low temperature start),the thrice fuel injection is selected to raise the exhaust gastemperature effectively. Onthe other hand, when the target exhaust gastemperature is set to a value lower than 400° C. but higher than 300°C., the twice fuel injection method is selected. Further, when thetarget exhaust gas temperature is set to a value lower than 300° C., theignition timing retard method is selected.

FIG. 22A shows the relationship between the indicated specific fuelconsumption rate ISFC (the abscissa) and the HC concentration (theordinate), and FIG. 22B shows the relationship between the indicatedspecific fuel consumption rate ISFC (the abscissa) and the exhaust gastemperature (the ordinate) both in low temperature start operation byway of example, in which the solid lines designate the method ofretarding theignition timing; the alternate long and short linesdesignate the method oftwice fuel injection; the dashed lines designatethe method of thrice fuel injection; and the alternate long and twoshort dashed lines designate themethod of throttling the intake pipe.FIGS. 22B indicates that the thrice fuel injection is effective to raisethe exhaust gas temperature to a relatively high value, as compared withthe twice fuel injection method and the ignition timing retardingmethod.

As shown in FIG. 22B, in this fifth embodiment, since the thrice fuelinjection is adopted when the engine 1 is started at a low temperature,itis possible to obtain the maximum exhaust gas temperature raisingeffect. Further, other than the low temperature starting, any one of thetwice fuel injection or the ignition timing retard is adopted to keepthe fuel consumption rate at a low level.

Further, when the temperature rasing method selecting section 72 selectsthe thrice fuel injection, the third injection amount setting section 76and the third injection timing setting section 7Y set the third fuelinjection amount T_(i3) and the third fuel injection timing T_(INJ3),respectively in addition to the second fuel injection amount T_(i2) andthe second fuel injection timing T_(INJ2) set by the second injectionamount setting section 62 and the second injection timing settingsection 63, respectively.

In this case, the third injection amount T_(i3) is set to equal to orslightly larger than the second injection amount T_(i2), and the thirdinjection timing T_(INJ3) is set at such an firing timing that thethirdinjection fuel can be injected into the cylinder immediately beforethe second injection fuel has been fired completely. In practice, thethird fuel injection timing T_(INJ3) is set between 90 and 120 degreesafter the top dead center, in comparison with the second fuel injectiontiming T_(INJ2) between 30 and 60 degrees after the top dead center.

In this fifth embodiment, after the first fuel injection setting routine(as already explained in the first embodiment with reference to FIG. 11)has been executed, the exhaust gas temperature raising routine as showninFIG. 21 is executed. That is, after the first fuel injection, any oneof the thrice fuel injection, the twice fuel injection and the ignitiontiming retarding is selected to raise the exhaust gas temperatureaccording to the setted target exhaust gas temperature.

In the exhaust gas temperature raising routine shown in FIG. 21, in stepS401, the control detects the engine operating conditions on the basisof the engine speed N and the basic fuel injection amount T_(p). In stepS402, the control detects the coolant temperature on the basis of thesignal of the coolant temperature sensor 15. Further, in step S403, thecontrol discriminates whether the engine starting conditions areestablished or not.

If the engine starting conditions are established, in step S404, thecontrol sets the target exhaust gas temperature. In step S405, thecontrolchecks whether the set target exhaust gas temperature is equal toor higherthan 400° C. or not. Further, if the engine starting conditionsare not established in step S403, the control proceeds from step S403 tostep S414 to set the target exhaust gas temperature.

In step S405, if the target exhaust gas temperature is lower than 400°C. (the ordinary engine starting), the control further checks whetherthe target exhaust gas temperature is equal to or higher than 300° C. Asa result, if 300° C. or higher in step S406, in step S407 the controlselects the twice fuel injection. In steps S408 and S409, the controlsets the second fuel injection amount T_(i2) and the second fuelinjection timing T_(INJ2), respectively, passing through theroutine. Iflower than 300° C. in step S406, in step S410 the control selects theignition retard. In step S411, the control sets the ignition timingretard rate θ_(RD), passing through the routine.

On the other hand, in step S405, if the target exhaust gas temperatureis equal to or higher than 400° C. (cool starting), the control proceedsfrom step S405 to step S412. In step S412, the control sets the secondfuel injection amount T_(i2) and the third fuel injection amount T_(i3).Further in step S413, the control sets the second fuel injectiontimingT_(INJ2) and the third fuel injection timing T_(INJ3), respectively,passing through the routine.

Accordingly, even when the engine 1 is started at a low temperature,since the exhaust gas temperature can be raised quickly, the catalyst 5can be activated fast, so that it is possible to remarkably reduceexhaust gas exhausted without purification.

Further, in the normal engine operation, in step S414, after the controlsets the target exhaust gas temperature, in step S415, the controlchecks whether the thrice fuel injection is necessary at the set targetexhaust gas temperature. If yes, the control proceeds to step S412 toset the second fuel injection amount T_(i2) and the third fuel injectionamount T_(i3). Further in step S413, the control sets the second fuelinjectiontiming T_(INJ2) and the third fuel injection timing T_(INJ3),respectively, passing through the routine. Further, in step S415, thecontrol discriminates that thrice fuel injection is not necessary forsuppression of fuel consumption rate, the control proceeds to steps S408and S409 to set the second fuel injection amount T_(i2) and the secondfuel injection timing T_(INJ2), respectively, passing through theroutine. As a result, it is possible to reduce the fuel consumptionrate, while activating the catalyst 5.

As described above, in the above-mentioned embodiments, since the secondfuel is injected from the early period to the middle period of theexpansion stroke of the first (primary) combustion in such a way thatthe second injection fuel can be fired by the flame propagation of theprimaryfuel, it is possible to fire the second injection fuel withoutdepending upon re-ignition (the possibility of mal-firing is large), sothat the exhaust gas temperature can be raised stably for catalystactivation.

Further, in the present invention, since any one of the twice fuelinjection and the ignition timing retard method (for retarding theignition timing of the first injection fuel) is selectively executedaccording to the engine operating conditions, and furthermore since thethrice fuel injection after the second fuel injection is selectivelyexecuted according to the activation condition of the catalyst 5, insuch a way that the third injection fuel can be fired by the flamepropagation of the second injection fuel, it is possible to realize theearly activation of the catalyst 5, while keeping the fuel consumptionrate at as low a level as possible.

Further, in the third or more additional fuels are injected into eachengine cylinder, after the primary combustion, according to the engineoperating conditions (e.g., as when an engine is started at a lowtemperature) and the catalyst characteristics (e.g., the target exhaustgas temperature is as high as 400° C. ), it is possible to activatethecatalyst 5 more quickly and effectively, while keeping the fuelconsumption rate at as low as possible.

(Sixth Embodiment)

The sixth embodiment will be described hereinbelow with reference toFIGS. 23 to 26. The feature of this sixth embodiment is to alwaysmonitor the combustion condition of the primary combustion in order toprevent the misfire of the second injection fuel for protection of thecatalyst 5. More in details, in the case where an abnormal (a low heatgeneration) combustion of the first (primary) injection fuel isdetected, the second fuel injection is interrupted. Further, the firstfuel combustion is shifted from the stratified combustion to the uniformcombustion, by changing the fuel injection timing, the fuel injectionamount and the ignition timing for the first injection fuel, in order toprotect the catalyst 5 from being deteriorated by raw gas.

Therefore, the sixth embodiment is different from the first embodimentshown in FIG. 6 in that the cylinder pressure sensor 9, a cylinderpressure detecting section 64, a heat generation rate calculatingsection 65, an abnormality discriminating section 66, a combustion modechanging section 67, an alarm generating section 68, a driver 69, and analarm device 80 are provided, in addition to the elements of the firstembodiment shown in FIG. 6.

More in details, the cylinder pressure detecting section 64 samples thesignals from the cylinder pressure sensor 9 for each predetermined timeperiod in response to a first trigger signal (the first fuel injectionsignal) outputted from the drive section 59 to the injector 7, tomeasure the inner pressure of the cylinder when the first injected fuelhas been ignited. The measured cylinder inner pressure is outputted tothe heat generation calculating section 65.

The heat generation calculating section 65 calculates the heatgeneration rate by the combustion of the first injection fuel on thebasis of the cylinder inner pressure measured by the cylinder innerpressure sensor 64 (after the first fuel injection) and the signalapplied by the crank anglesensor 13. The heat generation rate can becalculated with reference to a pressure-stroke diagram and on the basisof various parameters such as gasmass within the combustion chamber, thetemperature of the cylinder wall surface, the cylinder inner pressure,the ambient temperature and pressure, etc. Further, a thermal loss tothe outside is reduced from the calculated heat to obtain the quantityof heat generated by the first fuelcombustion.

The abnormality discriminating section 66 discriminates whether thecombustion of the first injection fuel is normal or not on the basis ofthe heat generation rate calculated by the heat generation calculatingsection 65. In the case where the combustion of the first fuel injectionamount T_(i1) is abnormal under the twice fuel injection conditions,theabnormality discriminating section 66 outputs a command indicative ofinterruption of the twice fuel injection to the injection timingcalculating section 57, and further resets the second fuel injectiontimer. In addition, the abnormality discriminating section 68 outputs anabnormal signal to the combustion mode changing section 67 to change thesucceeding combustion by the first (primary) fuel injection from thestratification combustion to the uniform combustion. Further, theabnormalsignal is outputted from the abnormality discriminating section66 to the alarm generating section 68 to activate the alarm device 80via the drive section 69.

In the abnormality discriminating section 66 for discriminating whetherthecombustion by the first fuel injection is normal or not, thecombustion abnormality can be discriminated by using heat generationdiagrams obtained for each cycle. In this case, the abnormality can bediscriminated in the cycle during which the generated heat is lower thanapredetermined reference value. On the other hand, it is possible to usea timing chart as shown in FIG. 26. More in details, when the heatgeneration rate at a predetermined crank angle in the expansion strokeof the primary combustion is smaller than the reference value Q_(REF)(at which the quantity of generated heat is large enough to fire thesecond injection fuel), the primary combustion is discriminated as beingabnormal.

Whenever the abnormality discriminating section 66 discriminates anabnormality, the combustion mode changing section 67 outputs acorrection command to the first injection amount setting section 54, thefirst injection timing setting section 55, and the ignition timingsetting section 56. In response to the correction command, the firstinjection amount setting section 54 corrects the first injection amountT_(i1) to an appropriate amount; the first injection timing settingsection 55 advances the angle of the first fuel injection timinglargely; and the ignition timing setting section 56 changes the fuelignition timing, so that the combustion mode of the succeeding fuelinjection can be changed from the stratified combustion to the uniformcombustion.

The operation of the ECU 20 will be explained with reference to a firstfuel injection set routine shown in FIG. 24.

First, in step S101, the control reads the basic fuel injection amountT_(p) on the basis of a predetermined address of the RAM 23. In stepS102, the control reads the engine operating condition parameters suchas the signals of the throttle sensor 12, the coolant temperature sensor15, and the like.

Further, in step S103, the control sets a coefficient COEF forcorrecting the basic fuel injection amount T_(p), which is determinedaccording to various engine operating conditions read in the step S102.In step S104, the control further sets the voltage correction rate T_(s)on the basis of a battery voltage V_(B), proceeding to step S111.

In the step S111, the control checks whether the combustion by theprimary fuel injection in the preceding cycle is normal or not, withreference to a combustion status discrimination flag F which is set andreset (cleared)by a combustion status detecting routine (describedlater).

If F=0, that is, if the combustion status is normal, the controlproceeds from the step S111 to the steps S106 and after setting thenormal fuel injection amount and the normal fuel injection timing. Onthe other hand, if F=1, that is, if the combustion status is abnormal,the control proceeds from the step S111 to the steps S108 and aftercorrecting the first fuel injection amount and timing.

That is, when the combustion of the primary injection fuel is normal inthepreceding cycle, in step S105, the control sets the first injectionamount T_(i1) (T_(i1) ←T_(p) ×COEF +T_(s)) by multiplying the basic fuelinjection amount T_(p) read in step S101 by the correction coefficientCOEF sat in step S103 and further by adding the voltage correction rateT_(s) set in step S104. Further, in step S106, the control sets thefirst fuel injection timing T_(INJ1) according to the combustion method,passing through the routine.

On the other hand, when the combustion of the primary fuel injection isabnormal in the preceding cycle, in step S108 the control reads thefirst injection amount correction value ΔK (which is set by thecombustionstatus detect routine (described later)) from the RAM 23 tocope with the abnormal combustion of the primary fuel injection. On thebasis of the read first injection amount correction value ΔK, the firstinjectionamount T_(i1) is corrected as (T_(i1) ←T_(p) ×COEF×ΔK+T_(s)),proceeding to the step S109.

In step S109, in the same way as above, the control reads the firstinjection timing correction value ΔTM (which is set by the combustionstatus detect routine (described later)) from the RAM 23 to cope withthe abnormal combustion of the first injection fuel. On the basis of theread first injection timing correction value ΔTM, the first injectiontiming T_(INJ1) is corrected as (T_(INJ1) ←T_(INJ1) -ΔTM), proceeding tothe step S110.

In step S110, the corrected first injection amount T_(i1) and the firstinjection timing T_(INJ1) are both set to the first fuel injectiontimer, passing through the routine.

Further, the second fuel injection setting routine is executed in quitethesame way as that shown in FIG. 12.

Upon end of the respective routines, the first injection timer isstarted when the cylinder to be injected reaches a predeterminedreference crank angular position (e.g., the engine top dead center), toinject the first fuel injection into the cylinder. In the same way, thefuel ignition timeris started when the cylinder to be ignited reachesthe predetermined reference crank angular position, to ignite themixture in the combustion chamber, to start the primary combustion bythe first fuel injection.

Here, the combustion status of the primary fuel by the first fuelinjectionis always monitored by the combustion status detecting routineas shown in FIG. 25.

In this combustion status detecting routine, in step S501, the controldetects the cylinder inner pressure. In step S502, the controlcalculates the heat generation rate. Further, in step S503, the controldiscriminateswhether the primary combustion of the first fuel injectionis normal or noton the basis of the heat generation rate. In this case,the heat generationrate diagram obtained for each engine cycle is used.Alternatively, it is also possible to discriminate the abnormality bychecking whether the heatgeneration rate at a predetermined crank anglein the expansion stroke of the primary combustion is high enough to firethe second fuel injection. If normal, the control proceeds to step S504to clear (F←0) the combustion status discrimination flag indicative ofthe normal primary combustion, passing through the routine.

Further, if the abnormality of the primary combustion by the first fuelinjection is discriminated, the control proceeds from the step S503 tostep S505 to generate an alarm to a driver. Further, in step S506,controlresets the second injection timer to interrupt the second fuelinjection. Further, in step S507 and after, the control sets respectivecorrection values for the first injection timing, the first injectionamount, and theignition timing in such a way that the primary combustionby the first fuelinjection can be changed from the stratified combustionto the uniform combustion.

More in details, in step S507, the control sets the first injectiontiming correction value ΔTM to change the stratified combustion to theuniform combustion by remarkably advancing the angle of the firstinjection timing T_(INJ1). Further, in step S508, the control sets thefirst injection amount correction value ΔK to optimize the firstinjection amount T_(i1). Further, in step S509, the control sets theignition timing retard angle θ_(RD) by retrieving from a map obtained onthe basis of the engine speed N and the basic fuel injection amountT_(p). Finally, in step S510, the control sets the combustion statusdiscriminate flag (F←1) indicative of the abnormal primary combustion,passing through the routine.

As described above, when the twice fuel injection conditions aresecurely satisfied and further when the first injection fuel injected atthe latterperiod in the compression stroke is ignited normally under thestratified combustion (in the low fuel consumption rate and the betterexhaust gas characteristics), the second fuel injection timer starts toinject the second fuel at such an optimum timing that the secondinjection fuel can be injected immediately before the primary combustionby the first injection fuel ends. As a result, the combustion by thesecond injection fuel can be started by the flame propagation of theprimary combustion, without need of re-ignition.

This second combustion is almost consumed to raise the exhaust gastemperature without moving the piston. In addition, since the secondinjection timing is optimized, it is possible to prevent the smoke frombeing generated due to the insufficient fuel diffusion or HC from beingincreased due to an excessive fuel diffusion, with the result that theexhaust gas temperature can be raised stably for activating the catalyst5.

On the other hand, when the primary combustion by the first injectionfuel is abnormal under the engine operating range in which the twicefuel injection conditions are satisfied, the second fuel injection isimmediately interrupted, so that it is possible to prevent raw gas frombeing exhausted as it is and further prevent the catalyst from beingoverheated or degraded or damaged, which would otherwise be caused bythe misfire of the second fuel injection. Further, since the fuelinjection timing of the primary combustion in the succeeding cycle islargely advanced and further since the combustion parameters such as thefuel injection amount and the ignition timing are changed, thecombustion mode of the primary fuel is changed from the stratifiedcombustion to the uniform combustion. As a result, although the fuelconsumption rate slightly increases, even if the abnormal combustionoccurs, it is possibleto operate the engine 1 in safe without anytrouble.

In addition, in the sixth embodiment, since the primary combustionstatus is discriminated on the basis of the heat generation rate at apredetermined crank angular position at the expansion stroke of theengine1, that is, by checking that the generated heat quantity is largeenough tofire the second injection fuel, any abnormality (e.g., amomentary abnormality of which frequency is very low) can be detectedand corrected,so that it is possible to increase the reliability of thecatalyst activation control system.

(Seventh Embodiment)

The seventh embodiment will be described hereinbelow with reference toFIGS. 27 to 28. This embodiment is different from the sixth embodimentin that when the primary combustion is abnormal (the heat quantity isnot sufficient), the amount of the primary combustion is increasedwithout changing the stratified combustion to the uniform combustion. Inmore detail, the feature of this embodiment is such that when asufficient heatquantity for firing the second injection fuel cannot beobtained by the primary combustion of the first injection fuel under theoperating conditions which satisfy the twice fuel injection, thequantity of heat ofthe primary combustion in the succeeding cycle isincreased to securely fire the second injection fuel, while keeping thestratified combustion (without changing to the uniform combustion).

Therefore, the ECU 20 of this embodiment further comprises a firstcombustion correction selecting section 81 for selecting parameters forcorrecting the primary combustion and a first combustion correctionsetting section 82 for setting the correction values of the selectedparameters, instead of the combustion mode changing section 67, inaddition to the elements of the sixth embodiment shown in FIG. 23.Further, in this embodiment, since no alarm is generated, the alarmrelated elements (the alarm generating section 68, the driver 69 and thealarm device 80) are omitted.

In this embodiment, when the abnormality discriminating section 66discriminates that the primary combustion status is abnormal; that is,theheat generation rate at a predetermined crank angular position in theexpansion stroke of the main combustion is lower than a reference valueQ_(REF) (at which the generated heat quantity is large enough to firethe second injection fuel), the abnormality discriminating section 66outputs a command indicative of abnormality to the first combustioncorrection selecting section 81 to increase the heat generation quantityof the primary combustion by the succeeding first fuel injection,without interrupting the second fuel injection and generating an alarm.

In response to the abnormality command, the first combustion correctionselecting section 81 selects changes of the parameters related to theprimary combustion. These parameters are an increment value of the firstfuel injection amount, the advancing or retarding angle of the ignitiontiming, the change of the first injection timing. In this case, a singleor a plurality of the parameters are selected according to the heatgeneration rate of the primary combustion. The first combustioncorrectionselecting section 81 outputs a command or commands indicativeof a selectedparameter or parameters to the first combustion correctionsetting section 82.

In response this command or these commands, the first combustioncorrectionsetting section 82 sets a correction value or correctionvalues of the selected parameters (e.g., the first fuel injectionamount, the ignition timing, the first injection timing. ) selected bythe first combustion correction selecting section 81, and outputs thecorrection value or values to the first injection amount setting section54, the first injection timing setting section 55, or the ignitiontiming setting section 56, to correct the combustion status of theprimary combustion, that is, to increase the heat generation. In thiscorrection, however, a predetermined allowable range is predeterminedfor each value. Therefore, when the heat generation cannot be increasedsufficiently within the allowable range, the correction is not executedbeyond the allowable range.

The operation of the combustion status detecting routine of the ECU 20of this seventh embodiment will be described hereinbelow with referenceto FIG. 28.

In the combustion status detecting routine, in step S601, the controldetects the cylinder inner pressure. In step S602, the controlcalculates the heat generation rate. Further, in step S603, the controldiscriminateswhether the primary combustion of the first injection fuelis normal or noton the basis of the heat generation rate. In this step,the abnormality of the primary combustion by the first injection fuel isdiscriminated whether the heat generation rate is high enough to firethe second injection fuel at a predetermined crank angular position ofthe expansion stroke of the primary combustion.

If normal; that is, if a heat quantity large enough to fire the secondinjection fuel can be obtained by the primary combustion, controlproceedsto step S604 to clear (F←0) the combustion status discriminateflag indicative of the normal primary combustion, passing through theroutine.

However, if abnormal; that is, if a heat quantity large enough to firethe second injection fuel cannot be obtained by the primary combustion,control proceeds from the step S603 to step S605 to select the parameteror parameters to improve the primary combustion, proceeding to stepS606.

In step S606, control sets the correction value or values of theparameter or parameters selected in step S605. Finally, in step S607,control sets the combustion status discriminate flag (F←1) indicative ofthe abnormal primary combustion, passing through the routine. Further,when the first injection amount and the first injection timing are bothnot corrected, in the first fuel injection amount setting routine shownin FIG. 24, the correction value is kept at 1 or 0 in step S108 or stepS109,without correcting any substantial correction.

In this embodiment, since the combustion status of the primarycombustion can be corrected (the heat quantity is increased) when a heatquantity large enough to fire the second injection fuel cannot beobtained by the primary combustion of the first injection fuel under theoperating conditions which satisfy the twice fuel injection conditions,it is possible to securely prevent the raw gas from being emitted as itis. Accordingly, the catalyst can be prevented from being overheated,degradedor damaged due to the misfire of the second injection fuel.

While the presently preferred embodiments of the present invention havebeen shown and described, it is to be understood that these disclosuresare for the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A method of activating a catalyst for a directfuel injection engine, comprising the steps of:detecting engineoperating parameters; calculating a basic fuel injection amount on thebasis of the detected engine operating conditions; checking whethern-time fuel injection is needed or not; setting a first fuel injectionamount by correcting the basic fuel injection amount on the basis ofvarious engine operating conditions and further setting a first fuelinjection timing when n-time fuel injection is not needed; and setting afirst fuel injection amount, a second fuel injection amount, a thirdfuel injection amount, in sequence when n-time fuel injection is needed;and further setting a first injection timing, a second fuel injectiontiming, a third fuel injection timing in sequence.
 2. The catalystactivation control system for a direct fuel injection engine forspark-igniting primary fuel directly injected into each cylinder so asto perform the stratification combustion while purifying exhaust gas bya catalyst disposed in an engine exhaust system, wherein an additionalfuel is injected at least once into each cylinder succeeding to theprimary fuel combustion so as to fire the additional injection fuel byflame propagation of the preceding fuel combustion when the catalyst isinactivated due to the stratification combustion.
 3. The catalystactivation control system according to claim 2, whereinany one of theadditional fuel injection method and an ignition timing retarding methodis selected in accordance with the engine operating conditions.
 4. Thecatalyst activation control system according to claim 2, whereinany oneof a method of changing a stratification combustion to a uniformcombustion, a method of increasing the first fuel injection amount, anda method of interrupting the additional fuel injection is selectivelyselected when a quantity of heat generated by the first fuel combustionis too low to fire the additional injection fuel.
 5. A method ofactivating a catalyst for a direct fuel injection spark ignition enginehaving an injector and a spark plug disposed in a combustion chamber ofeach cylinder and using stratification combustion and uniformcombustion, comprising the steps of:detecting various engine operatingconditions; performing primary combustion by spark igniting primary fueldirectly injected into each cylinder, the amount of fuel beingcalculated depending on said engine operating conditions; and performingsubsidiary combustion by injecting subsidiary fuel into each cylinderduring the expansion stroke of said primary combustion so as to firesaid subsidiary fuel by flame propagation of said primary combustionwithout spark-igniting, whereby an exhaust gas temperature can be raisedfor catalyst activation.
 6. The method of activating the catalystaccording to claim 5, wherein the subsidiary combustion is performed ata crank angle between 30 and 60 degrees after top dead center.
 7. Themethod of activating the catalyst according to claim 5, furthercomprising the steps of:detecting a pressure of each cylinder;calculating a heat generation rate depending on the detected pressure;determining whether the calculated heat generation rate is normal orabnormal; and executing the subsidiary fuel injection when thecalculated heat generation rate is normal and interrupting thesubsidiary fuel injection and correcting a parameter related to theprimary combustion when the calculated heat generation rate is abnormal.8. The method of activating the catalyst according to claim 7, whereinsaid parameter related to the primary combustion is any one of theprimary fuel injection timing, the primary fuel injection amount, andthe ignition timing.
 9. The method of activating the catalyst accordingto claim 5, further comprising the steps of:determining whether saidtarget exhaust gas temperature is higher than a first predeterminedvalue; and selecting to perform subsidiary combustion according to thesubsidiary fuel injection to raise the exhaust gas temperature when saidtarget exhaust gas temperature is higher than said first predeterminedvalue and selecting to perform the ignition retard without thesubsidiary combustion to raise the exhaust gas temperature when saidtarget exhaust gas temperature is lower than said first predeterminedvalue.
 10. The method of activating catalyst according to claim 9,wherein said first predetermined value is 300° C.
 11. The method ofactivating the catalyst according to claim 9, further comprising thesteps of:measuring an exhaust gas temperature; determining whether themeasured exhaust gas temperature reaches said target exhaust gastemperature; determining whether the subsidiary combustion is selectedor the ignition retard is selected to raise the exhaust gas temperaturedepending on the target exhaust gas temperature; and increasing anamount of the subsidiary fuel injection when the subsidiary combustionis selected and increasing a retard angle of the ignition timing whenthe ignition retard is selected.
 12. The method of activating thecatalyst according to claim 9, further comprising the stepsof:determining whether said target exhaust gas temperature is higherthan a second predetermined value; and selecting to perform the furthercombustion by injecting further fuel succeeding to the subsidiarycombustion so as to fire the injected fuel by flame propagation of thesubsidiary combustion without spark-igniting when said target exhaustgas temperature is higher than said second predetermined value.
 13. Themethod of activating the catalyst according to claim 9, wherein saidpredetermined value is 400° C.
 14. The method of activating the catalystaccording to claim 5, further comprising a step of:discriminatingwhether the current engine operation requires subsidiary fuel injectionfor the activation of the catalyst on the basis of the engine operatingconditions to permit subsidiary fuel injection.
 15. The method ofactivating the catalyst according to claim 14, wherein subsidiary fuelinjection is required when the engine operation is within apredetermined range of stratification combustion where the catalyst ispossibly inactivated.
 16. The method of activating the catalystaccording to claim 15, wherein said predetermined range is where theengine speed and the engine load are both low.
 17. A catalyst activationcontrol system for a direct fuel injection into a spark ignitionmulticylinder engine having a catalyst disposed in an engine exhaustsystem for said engine, said injection being into a combustion chamberof each cylinder for selectively performing a stratification combustionand uniform combustion, comprising:engine operating condition detectingmeans for detecting various engine operating conditions; primary fuelcombustion means for performing a primary combustion by spark-igniting aprimary fuel directly injected through an injector into each cylinderdepending on said engine operating conditions; and catalyst activatingmeans for injecting a subsidiary fuel into each cylinder during theexpansion stroke of the primary fuel combustion so as to fire thesubsidiary injection fuel by flame propagation of the primary fuelcombustion without spark-igniting whereby an exhaust gas temperature canbe raised for catalyst activation.
 18. The catalyst activation controlsystem according to claim 1, wherein said primary fuel combustion meanscomprises:combustion mode deciding means based on the engine operatingconditions detected by said engine operating condition detecting meansfor deciding the combustion mode on any one of an ordinary uniformcombustion for injecting fuel in an early period of an engine intakestroke and a stratification combustion for injecting fuel in a laterperiod of the compression stroke but immediately before fuel sparkignition; primary fuel injection amount setting means for setting aprimary fuel injection amount in accordance with the current engineoperating conditions; primary fuel injection timing setting means forsetting a fuel injection timing for said calculated primary fuelinjection amount in accordance with the combustion mode and the engineoperating conditions; ignition timing setting means for setting anignition timing by correcting a basic ignition timing decided by thedetected engine operating conditions; fuel injection timing calculatingmeans for calculating the set fuel injection timing in terms of a crankangular position relative to a reference crank angle, and outputting thecalculated fuel injection timing signal to said injector; and ignitiontiming calculating means for calculating the set fuel ignition timing interms of a crank angular position relative to the reference crank angleposition and for outputting the fuel ignition timing signal to saidspark plug.
 19. The catalyst activation control system according toclaim 18, whereinsaid primary fuel injection timing setting meansdecides a fuel injection start timing in the case of the ordinaryuniform combustion and a fuel injection end timing in the case of thestratification combustion.
 20. The catalyst activation control systemaccording to claim 17, wherein said catalyst activating meanscomprises:subsidiary fuel injection discriminating means fordiscriminating whether the current engine operation requires a pluralityof subsidiary injections for the activation of the catalyst on the basisof the engine operating conditions and for outputting a commandindicative of plural subsidiary fuel injections; subsidiary fuelinjection amount setting means responsive to the plural subsidiary fuelinjection command for setting a plurality of subsidiary fuel injectionamounts; and subsidiary fuel injection timing setting means for settinga plurality of fuel injection timings for the respective subsidiary fuelinjection amounts so that the subsidiary fuel in each occurrence isfired by the flame propagation of the preceding fuel combustion, aplurality of the set fuel injection timings being applied to said fuelinjection timing calculating means.
 21. The catalyst activation controlsystem according to claim 20, wherein said subsidiary fuel injectiontiming setting means adapted to set the injection timing for the firstsubsidiary fuel between 30 and 60 degrees after a top dead center in theprimary fuel combustion, and the injection timing for the secondsubsidiary fuel between 90 and 120 degrees after top dead center in thefuel combustion of the first subsidiary fuel.
 22. The catalystactivation control system according to claim 17, wherein said catalystactivating means further comprising:subsidiary fuel injectiondiscriminating means for discriminating whether the current engineoperation requires the subsidiary fuel injection for the activation ofthe catalyst on the basis of the engine operating conditions and foroutputting a command indicative of the subsidiary fuel injection;subsidiary fuel injection amount setting means responsive to thesubsidiary fuel injection command for setting an injection amount of thesubsidiary fuel; and subsidiary fuel injection timing setting means forsetting a fuel injection timing for the subsidiary fuel injection amountso that the subsidiary fuel is fired by the flame propagation of theprimary fuel combustion, the set subsidiary fuel injection timing beingapplied to said fuel injection timing calculating means.
 23. Thecatalyst activation control system according to claim 22, whereinsaidsubsidiary combustion discriminating means discriminates that thesubsidiary fuel injection is required when the engine operation iswithin a predetermined range of the stratification combustion where thecatalyst is possibly inactivated.
 24. The catalyst activation controlsystem according to claim 22, whereinsaid subsidiary fuel injectionamount setting means sets an injection amount of the subsidiary fuel tobe fired by the flame propagation of the primary fuel combustion on thebasis of the basic fuel amount.
 25. The catalyst activation controlsystem according to claim 22, whereinsaid subsidiary fuel injectiontiming setting means sets an injection timing for the subsidiary fuelinjection amount with reference to a map decided empirically on thebasis of the detected engine speed and the engine load represented bythe calculated basis fuel injection amount engine operation conditions.26. The catalyst activation control system according to claim 22,whereinsaid subsidiary fuel injection timing setting means is adopted toset said subsidiary fuel injection timing between 30 and 60 degreesafter a top dead center in the primary fuel combustion.
 27. The catalystactivation control system according to claim 22, furthercomprising:cylinder pressure detecting means for detecting a cylinderpressure generated by the first fuel combustion; heat generation ratecalculating means for calculating a heat generation rate by the primaryfuel combustion on the basis of at least said cylinder pressure;abnormality discriminating means for discriminating whether the primaryfuel combustion is normal or abnormal on the basis of the calculatedheat generation rate; correcting parameter selecting means for selectingat least one parameter related to the primary fuel combustion when saidabnormality discriminating means discriminates that the primary fuelcombustion is abnormal; and correcting parameter setting section forsetting a correction value of the selected parameter so as to increasethe heat generation rate by the primary fuel combustion in thesucceeding cycle without changing the combustion mode from thestratification combustion to the uniform combustion.
 28. The catalystactivation control system according to claim 27, whereinsaid at last oneparameter is any one or a combustion of an increase rate of the primaryfuel injection amount, an advance or retard angle of the ignitiontiming, a displacement of the primary fuel injection timing each beingset within a predetermined allowable range.
 29. The catalyst activationcontrol system according to claim 27, whereinthe subsidiary fuelcombustion is interrupted in the same cycle when said abnormalitydiscriminating means discriminates that the primary fuel combustion isabnormal.
 30. The catalyst activation control system according to claim22, further comprising:cylinder pressure detecting means for detecting acylinder pressure generated by the primary fuel combustion; heatgeneration rate calculating means for calculating a heat generation rateby the primary fuel combustion on the basis of at least said cylinderpressure; abnormality discriminating means for discriminating whetherthe primary fuel combustion is normal or abnormal on the basis of thecalculated heat generation rate; and combustion mode changing means forchanging the stratification combustion to the uniform combustion whensaid abnormality discriminating means discriminates that the primaryfuel combustion is abnormal.
 31. The catalyst activation control systemaccording to claim 30, whereinsaid heat generation calculating meanscalculates the heat generation rate on the basis of a pressure-strokediagram obtained by setting various engine operating conditions asparameters.
 32. The catalyst activation control system according toclaim 30, whereinsaid abnormality discriminating means decides theabnormality when the generated heat is lower than a predetermined valuewith reference to a heat generation diagram obtained for each enginecycle.
 33. The catalyst activation control system according to claim 30,whereinsaid abnormality discriminating means decides the abnormalitywhen the heat generation rate at a predetermined crank angular positionin the expansion stroke according to the primary fuel combustion islower than a reference heat quantity large enough to fire the secondinjection fuel.
 34. The catalyst activation control system according toclaim 30, further comprising:alarm generating means for generating analarm to a driver in case of abnormality.
 35. The catalyst activationcontrol system according to claim 22, further comprising:target exhaustgas temperature setting means for setting a target exhaust gastemperature when said subsidiary fuel injection discriminating meansdiscriminates that the engine operation requires the subsidiary fuelinjection to activate the catalyst; temperature raising method selectingmeans for selecting one of a subsidiary fuel injection method and anignition timing retard method in accordance with the set target exhaustgas temperature; and ignition timing retarding means for retarding theignition timing set by said ignition timing setting means without thesubsidiary fuel injection when said temperature raising method selectingmeans selects the ignition timing retard method.
 36. The catalystactivation control system according to claim 35, whereinsaid targetexhaust gas temperature setting means sets the target exhaust gastemperature on the basis of catalyst characteristics and the engineoperating conditions.
 37. The catalyst activation control systemaccording to claim 35, whereinsaid temperature raising method selectingmeans selects the ignition timing retard method when the set targetexhaust gas temperature is about 300° C. and but the twice fuelinjection method when the set target exhaust gas temperature is 300° C.or higher.
 38. The catalyst activation control system according to claim35, whereinan ignition timing retard rate is determined with referenceto a map decided on the basis of engine speed and the basic fuelinjection amount when the ignition retard method is selected.
 39. Thecatalyst activation control system according to claim 35, furthercomprising:an exhaust gas temperature sender; and comparing means forcomparing the target exhaust gas temperature set by said target exhaustgas temperature setting means with an actual exhaust gas temperaturedetected by said exhaust gas sensor to produce a difference; whereinsaid subsidiary fuel injection amount setting means adjusts the setsubsidiary fuel injection amount based on said difference when saidtemperature raising method selecting means selects the subsidiary fuelinjection method, and said ignition timing retarding means adjusts theignition timing retard rate based on said difference when saidtemperature raising method selecting means selects the ignition timingretard method.
 40. The catalyst activation control system according toclaim 35, further comprising:an exhaust gas temperature sensor; catalystactivation discriminating means for discriminating whether an actualtemperature detected by said exhaust gas temperature sensor reaches saidtarget exhaust gas temperature; further fuel injection amount settingmeans for setting an amount of the further fuel injection succeeding tosaid subsidiary fuel injection when said target exhaust gas temperatureis set in a predetermined range; and further fuel injection timingsetting means for setting an injection timing for said further fuelinjection amount to be fired by the flame propagation of the subsidiarycombustion.
 41. The catalyst activation control system according toclaim 40, whereinthe thrice fuel combustion is selected when the settarget exhaust gas temperature is 400° C. or higher at cool enginestart; the twice fuel combustion is selected when the set target exhaustgas temperature is between 300° C. and 400° C.; the ignition timingretard is selected when the set target exhaust gas temperature is 300°C. or lower.